"It's a genetic anomaly - a one in a million chance - which in this case probably had no significance for the animal."
A veterinary surgeon has now removed them, putting an end to any plans McLeod might have had for an exciting climax to the millennium.
The third testicle was just three-quarters the size of the other two. And fearing it might be cancerous, the vet from Ely in Cambridgeshire, UK, sent all three objects for tests at the nearby Animal Health Trust's diagnostic facilities in Newmarket.
But pathologist Ken Smith reported a trio of perfect specimens. "My part in all this was to confirm that all three structures were indeed testicles," he told BBC News Online.
"The curious thing was that they were all otherwise quite normal. They were all connected up correctly, so it would appear that they would have been fully functional - the cat would have been fertile prior to the surgery."
Literature search
Because neither Smith nor Milwright had seen such a case before, they did a search of veterinary literature. But they were surprised to find no similar case reported anywhere else in the world.
"It's one of these conditions that is extremely rare," said Ken Smith. "It's picked up occasionally in farm animals - particularly in piglets, calves and foals - but to our knowledge, it hasn't previously been reported in cats.
"It's a genetic anomaly - a one in a million chance - which in this case probably had no significance for the animal."
The two scientists reported their findings in the Veterinary Record.
As for McLeod, he seems unbothered by stardom. "He's a very friendly, loving cat that likes to lie on your lap all day," said owner Sarah Bales, who has had the animal since birth.
"He's never been one for outside. He wasn't really a ladies man anyway - he wasn't in to all that. He's just a bagpuss - an old, saggy cloth cat."