I was looking for these odds and couldn't find them, so I decided to try and work them out myself. I know it's an old thread but maybe there are other people looking for them too.
I like to look at these problems the other way around. Let's say it's a heads up scenario. If your opponent DOESN'T have those trips (or quads), then (s)he must have a hand made from the other cards in the deck. So, they have 45 choices out of the 47 remaining cards for their first card, and 44 out of 46 for the second. These odds work out to (45/47)*(44/46) = 91.6%. So, the odds that your opponent DOES have at least trips (using the board pair) is 1 - 91.6% = 8.4%.
For more than one opponent, you can't just double the odds. For two opponents, the odds that neither one of them has either card that matches the board pair is (45/47)*(44/46)*(43/45)*(42/44) = 83.5%, so the odds that at least one of them has at least trips is 1 - 83.5% = 16.5%.
I tabulated these odds for a number of opponents below.
1 opponent - 8.4%
2 opponents - 16.5%
3 opponents - 24.1%
4 opponents - 31.5%
5 opponents - 38.4%
6 opponents - 45.0%
7 opponents - 51.2%
8 opponents - 57.0%
9 opponents - 62.4%
10 opponents - 67.5%
Of course, someone may have folded one of those matching cards pre-flop, so you should only count the people who are remaining in the hand.