Leonardo DiCaprio, Robert De Niro, Robert Downey Jr. Dine at Giorgio Baldi

Leonardo DiCaprio, Robert De Niro and Robert Downey Jr. walk into a restaurant ... there's no punch line here -- 'cause it really happened Thursday night!

The three Oscar winners all hit up Giorgio Baldi -- a fancy Italian spot in Santa Monica ... with the Roberts enjoying a night out with their significant others and Leo rolling in solo.

All three men dressed casually ... nice jackets and T-shirts -- nobody went full suit. De Niro and Downey were easily identifiable, not covering up at all -- while Leo wore his usual hat and mask to avoid giving photogs a shot of his face.

We're told this was no star-studded coincidence -- they met and had dinner together at the same table.

De Niro and DiCaprio are longtime collaborators -- first appearing onscreen together back in the 1993 movie "This Boy's Life" when Leo was just 19. They also starred in the Oscar-winning flick "Killers of the Flower Moon" last year.

Downey, on the other hand, hasn't worked with either of the men ... though he collected his first-ever Oscar earlier this year for his role in "Oppenheimer." He's headed back to the Marvel Cinematic Universe, btw.

Whether they met to discuss business or just enjoyed a social night out together, one thing's for sure ... the other restaurant guests were treated to a red carpet show!