The prosecution claims that the UK-based trio coordinated espionage activities in several European countries.
A court in London has heard that three UK-based Bulgarian nationals allegedly passed secrets to Russia for nearly three years after carrying out surveillance across Europe on individuals and places of interest to Moscow.
Prosecutors at the Central Criminal Court on Thursday said Katrin Ivanova, 33, Vanya Gaberova, 30, and Tihomir Ivanov Ivanchev, 39, were part of a spy ring between 2020 and 2023.
The three defendants deny a charge of conspiracy to spy between 30 August 2020 and 8 February 2023.
Ivanova has denied a second charge of possession of false identity documents with improper intention.
The alleged spy ring also included two more defendants, Orlin Roussev and Biser Dzhambazov, who have previously pleaded guilty to espionage charges, the court was told.
While it is alleged much of their activity took place across Europe, including in cities such as Vienna and Valencia, the prosecution claims that their efforts were coordinated in the UK.
Among other allegations, the defendants are accused of spying on a US airbase in Germany where Ukrainian troops were said to be in training. The prosecution also contend that the defendants discussed kidnapping or killing opponents of the Kremlin.
"Between 2020 and 2023 these three defendants together with a number of other people spied for the benefit of Russia," said prosecution lawyer Alison Morgan. "Their activity caused obvious and inevitable prejudice to the safety and interests of the United Kingdom."
Morgan said the alleged surveillance was "sophisticated" and that the defendants were paid "significant amounts of money." Hundreds of devices are alleged to have been used, including hidden bugs and jammers.
It is alleged that the defendants plotted with a Russian agent, said to be Austrian national Jan Marsalek, to obtain information useful to Russia.
According to both German and Austrian media, Marsalek is believed to have had connections to Russian intelligence since at least 2014, and is widely believed to be in Russia.
The trial is expected to last until February.