For those unfamiliar with the terminology, each player gets two cards to himself and the three flop cards are shared among all players. So this is the same as asking if you dealt three community cards, all of different ranks, and ten 2-card hands, what is the probability three of the 2-card hands would be pairs that match one of the three community cards.
The last set of clues that I look for when considering if I should fold my overpair are the history that I have with this person and any reads that I may have gathered in the past. If you encounter a player at the micros who is sophisticated enough to be able to bluff-raise or semi-bluff raise your double barrel for instance, they are much more likely to do it when there is significant history.
An overpair is a pair of downcards of equal rank (a pocket pair) which are higher than any exposed upcards in a hand of poker. The most common usage is in a game like Hold'em, when a player has a pocket pair such as KK, and the flop comes J-8-7. In this case, the player has a pair that is higher than any card on the board, so it is called an overpair.
First off, as mentioned, getting set over set is an extremely rare occurrence in poker. In fact, the math suggests that somebody will flop a higher set than you in a full ring cash game once every 7.8k hands. It is even more rare in a 6max game, where it will only happen once every 12.6k hands. This is why set over set is commonly referred to.
