This seems like one of those problems you solve in math class which has little application to the real world. Even if you do know the exact probability for any player, each specific situation would have a different probability depending on your read of the player. For example if a player limps in and/or flat calls a small preflop raise, they're more likely to have a small pocket pair hoping to flop a set. If they're raising and reraising on the flop, chances are they have a set. And if the board pairs by the river and they're still aggressive, they likely have a boat. While it's good to know the absolute probability someone has a boat, you need to put your opponents on a range based on their betting and previous play, and from that, figure out the probability they have a boat.
Anyways, the probability of the board pairing is 49.29%. The probability of one person getting a pocket pair is 5.9%. The probability of the pocket pair improving to a set by the river is 22%. So it's a 1.29% chance they have a boat when the board is paired. Multiply 1.29 by the number of players.
Sure, it's a low probability, but you have to use the information you have by observing how they're playing the hand to figure if they have a boat or not.