You dont have great implied odds here preflop. You have $50 and the bet is $7 to you. True, if you hit a set and got BOTH of them all in you would hve a nice payoff, but thats very unlikely.
You would need a perfect flop for that to happen. Something like [5d][3s]. If they have [Ks] and [Ac], you might take both stacks, but if they help those cards, the first raiser wouldve reraised preflop, probably all in since you came along.

Its more likely that the first raiser has something like [Qd] and the second guy [Ks].

Then theres the chance that you hit a flop like this one and now your jacks look better to you than they really are. Its much tougher to fold jacks to a rag flop than it is to fold them preflop.

I wouldve folded preflop unless the stacks were much bigger. At Empire, I would be folding that almost every time. At other sites with 100BB stacks its a different story.

I realize that by checking I was giving up the hand, which I had when I saw I didn't hit my set, but the bet size really through me for a loop.

Iceman, I heard somewhere that when you are trying to hit a set your opponent should have at least 10 times the amount you are calling. I had posted the $1 BB so it was $6 to complete and my opponent had a little over $60 in his stack. Maybe I wouldn't get too much money out of the initial raiser, but the fact that a third player was in raised my implied odds a little bit. If my opponent having 6 times what I'm calling in his stack isn't a good enough indicator of implied odds, how do you judge them?

Another thing to consider is whose action will close the betting for the round. You would always prefer it be yours, but in this case it will be the original raiser. If the original raiser decides to do some more raising preflop, you will have to abandon your $6 additional you put in there.
I like to use the 15x rule for trying to flop a set on someone. If you can win 15 or more times the amount you have to call with your pair to see the flop, then it is good. Less than 10 is a muck. In this case, you can win about about 7.5 times what you have to call, not quite enough.
This is one of the annoyances of sitting at table with multiple short stacks (I know these two were not, but..). You can't win enough to make it worth playing, and often the guys with the short stacks like to frequently push them all-in preflop.
Mike