Don't go with the 50/50 rule, it is only roughly 50/50 they will get a higher pair than you pocket pair, if they have to 'over' cards, please remember straights are also more likely if you have 2 different cards, flushes will be more likely if they have suited cards, also if 4 of a suit comes out, it is more likely that they will have a higher suited card.
Not forgetting that if 2 pairs come out, chances are that you are suddenly left with the lowest kicker. Low pocket pairs rarely have a 50/50 chance of winning any sort of hand that would call a big raise, so although a raise can be beneficial, i would be wary of going all in, or even making a large raise, if that 3 of a kind doesn't turn up (and chances are that a vast majority of the time it won't) then bail those cards cards if it looks like anyone else has hit a higher pair.
Never hold on for the turn or river to give you 3 of a kind, it just won't happen nearly often enough to work