Question: Why were only three predators sent to deal with all the aliens?
Answer: The Aliens are trophies for the Predators, not some ancient mortal enemy. The 3 Predators are a literal hunting party sent for sport. This is why Danny Glover is not killed at the end of Predator 2 as he had managed to hunt and kill a Predator to its death.
Question: Can anyone tell me why the Predators didn't scan the 'token' human survivor or the last dead predator to see if either were infected with an Alien? Especially when the Predator was not wearing his face mask which would serve as protection against this infection.
Answer: When they emerged from the underground pyramid unscathed, the Predators assumed they were not infected. Any Predator, young or old, would have been taught to self destruct rather than be caught or killed. As seen in the previous movies. Lex was given the mark of a warrior, they assumed she was taught their code of honor.
Chosen answer: Given the speed with which the chestbursters emerged from the other humans, it seems likely that these have been engineered by the Predators to emerge as quickly as possible. As such, they probably assumed in Lex's case that, were she impregnated, it would have emerged before that point. As for their fallen comrade, it may well have simply not occurred to them that one of their number would be foolish enough to get himself impregnated. Even proud alien hunters can make mistakes.
Answer: As the film explains, it's part of a "rite of passage" ritual where a small group of chosen predators are sent to hunt xenomorphs to prove themselves worthy. The xenomorphs are the "ultimate prey" for them to hunt. There's only three because that creates more of a challenge for them to overcome. If there were more predators, it wouldn't be as much of a challenge, and thus it wouldn't really prove them to be "worthy."
TedStixon