if all cards were known, this leaves 46 cards unaccounted for..
C(46,3)=15180
the flop must include one of the remaining aces and one of the remaining jacks, leaving 44 cards
1*1*44
15180/44 = 345
so approx 344 to 1 against
unknown cards pre-flop i"d guess the easiest way of working the probability out then would be to multiply the odds 344/1 by the chance that two players at an X-handed table are dealt pocket pairs by the chance another player has one of each AND by the chance that they all see the flop...