well, it's when they're talking about who the judges will be, and Hermione says that the three heads were to be judges since they were all injured in the last one, over 200 years ago (she gives a year).
grrrr just a second, lemme go look...
okay, here we go...
"Who are the judges?" Harry asked.
"Well the Heads of the participating schools are always on the panel," said Hermione, and everyone looked around at her, rather surprised, "because all three of them were injured during the Tournament of 1792, when a cockatrice the champions were supposed to be catching went on a rampage."
So, no, it doesn't specifically say that Dumbledore, Maxime and Karkaroff were the three that got injured, but that's how I read it. I suppose it could be that the heads were automatically given the job of judging, and that tradition stuck around, because of the rampaging cockatrice in question...