My toxic triangle's bitchy fallout proves why three women can NEVER be friends: LISA TAYLOR

There can't be many people who haven't heard of a 'love triangle' and are not aware of the damage one can cause.

It's widely accepted that adding a third person to a romantic relationship spells disaster – whether as a betrayal or, at the very least, something kinky.

But there's another kind of triangle, the female friendship kind, that can be every bit as toxic and, from experience, far removed from the sisterly, wholesome and fun image it is often portrayed as.

I have a long-standing friendship with two women whom I met at our small private girls' school in London 40-odd years ago. There weren't many pupils in our tiny class, and we fell into our friendship, over the years enjoying weekend shopping trips, clubbing and parties.

Maybe it's telling that two of us – me and Belinda* – chose to sit side by side at our double desk in school, while the other, Caroline*, had to sit with someone we deemed too dull and frumpy to join our gang. Did this unwittingly set the tone for the two-plus-one type of relationship we've had ever since?

There has always been an undercurrent of competitiveness between us. Caroline always regarded herself as the prettiest. At 15, she was the first to get a boyfriend, and often boasted about how they would marry and his wealthy family would buy them a house.

However, she was dumped several years later, and when Belinda, at the age of 23, became the first of us to get engaged, Caroline burst into tears and later told me she didn't understand how it could happen to 'someone like her rather than me'.

Caroline went on to work in finance in the City and embarked on an affair with her boss, which accelerated her career in a way that Belinda and I agreed was most unfair when we discussed her behaviour at length. The truth was, we envied her now eye-watering salary more than we disapproved of her morals.

We are now all in our 50s and do our best to stay youthful. We fight the grey with regular hair appointments and the flab with expensive gym memberships. Right now, I'm the slimmest, but it hasn't always been that way.

A few months ago Caroline turned up for a drinks party, clutching a dress that she loudly announced was 'far too big for me, but will be perfect on you'.

What I don't tell either of my friends is that this bitchy comment spurred me to go from a size 14 to a ten with the help of weight-loss jabs. I pretended it was down to running, which I hardly ever do.

I can't quite believe that we are still competing so pettily in our 50s, but here we are.

The reality is our lives have diverged drastically as we've aged. We currently live more than 100 miles apart. Belinda has had a successful career in TV but struggled with infertility while Caroline never wanted children and has recently embarked on a passionate marriage to a much younger man. I have two grown-up sons and am desperate for grandchildren.

Belinda lives in London, I'm in the suburbs and Caroline has retired to a large house by the sea on her investments. Belinda and I still have to work – no wonder things are complicated.

It's why, when I settled down on the sofa, glass of wine in hand, to watch the third series of Sky's drama The White Lotus, which exposes the dark side of the wealthy wellness world, I found myself cringing in recognition before feeling distinctly uneasy.

The show's depiction of the paranoid, competitive, and often cruel dynamics of a female friendship triangle felt all too familiar.

The storyline features three childhood friends, now middle-aged, Botoxed, and bottle-blonde, 'enjoying' a girly reunion in Thailand. Kate is a rich housewife, Jaclyn is a famous TV actress, and Laurie is a lawyer and single mother whose life isn't quite as glossy as that of her friends. It doesn't take long for old rivalries to resurface – and the passive-aggressive comments to start flying.

Anyone observing our own triangle lunching at an expensive restaurant, as we do several times a year – kissing and hugging as we arrive, laughing as we order a second bottle of rose – might think everything was, well, rosy.

But if they looked more closely at what happens when one of us pops to the loo or noticed how we eye each other's plates to ensure no one is 'the greedy one', they'd get a very different impression.

I'm not proud to admit that in my triangular friendship, the minute Caroline is out of earshot Belinda and I are likely to lean in and ask each other an 'innocent' but leading question like, 'She looks great… but is that shade of blonde really working for her?' After lunch, Belinda and I are likely to text each other, voicing our 'concerns' before going full mid-life Mean Girls. 'I couldn't believe how she flirted with that waiter. It was so embarrassing!'

'What has she done to her face? Is that filler, or has she just put on weight?' We know we're being catty, but the guilt only adds to the adrenaline rush.

In this triangle, it's not me who is the 'third friend'. And I admit that hugely boosts my ego. In some ways, especially financially, I'm the poor relation of the trio, but my complicity with Belinda makes me feel important. Even more so because she has a glamorous career and a roster of celebrity friends that I know Caroline would love to cultivate.

In fact, Belinda often mocks Caroline's social-climbing ways. I know that if Queen B was to jettison anyone from our trio, it would be Caroline and not me, which makes me feel deliciously smug. In fact, sometimes I think that when I'm sharing unpleasant gossip with Belinda, part of my motivation is to stay in her orbit as I'm secretly terrified of being ostracised myself. What makes my behaviour even worse is that I've been in Caroline's position.

After having my first child, I was lonely and isolated at home. So I joined a National Childbirth Trust group and fell in with two women, 'Sarah' and 'Penny', who had babies the same age. Both were successful professionals but didn't know each other before becoming mothers, which I naively assumed would make us equals.

At first, I was delighted to have found a gang of yummy mummies. But I soon noticed the awkward silences when I returned from paying for my coffee, and one day caught them together in the park when they'd both claimed to be too busy to meet. Eventually, I discovered from another mother that they'd planned a weekend away without me and didn't want me to know about it. It shattered my self-esteem.

So why do I behave this way, knowing first-hand how hurtful it is? I wonder the same thing.

Perhaps we're incapable of spreading our affection and attention equally among two people. Maybe we seek the comfort of one trusted friend, and a third feels like emotional multi-tasking. Whatever the reason, trio friendships often come with competition and subtle ganging up.

Yet the glue of friendship is often more lasting than that of a love affair. In a love triangle, it's almost inevitable that the third party will either run off with the prize, or will be ditched. But friendships like mine can carry on for decades, with the one who suspects she's at the bottom of the pecking order paranoid and petrified of being surplus to requirements.

I believe this is an exclusively female dynamic. Women often mock male friendships as shallow compared to our deeper, more emotional bonds, but I'm afraid that bonding often means having someone to gossip with – and someone to gossip about.

My husband has overheard me dissecting a friend's flaws and asked: 'Why do you keep seeing her if you don't like her?' It's a fair question. The truth is, gossiping, and even downright character assassination, is a hobby for many women as Saturday morning football is for men.

Interestingly, in this particular friendship, whenever we see one of the others individually we have a perfectly lovely time. This makes me think that something about the trio itself is toxic, especially with old friends. Sometimes I worry that without a third wheel to scapegoat, my friendship with Belinda might fall apart.

Watching The White Lotus felt like holding up a mirror to my life. Maybe the solution is to stop pretending that a triangle is a healthy shape for a friendship. Or perhaps I can see it as a masterclass in why we should cut out the bitchiness and treat our female friends with respect.

Oh, who am I kidding? I know from experience that I never will. Like a drug, my toxic triangle is horribly bad for me, but it's also thrillingly addictive. 

• Lisa Taylor is pseudonym.

• All names and identifying details have been changed.