3rd nut low draw. A4 is the second nuts. And he has the straight made on the flop.
Your hand is certainly on the "not very good" end of things, but with 3 limpers, I'm certainly not folding it in the SB, especially in a pot limit game.
The funny thing is, even though you flopped the nuts, you're actually behind:
ProPokerTools Omaha Hi/Lo Simulation
820 trials (Exhaustive)
board: 3:club: 7:spade: 6:diamond:
Hand |Pot
equity
2c 4d 5d Qh| 42.62%
2s Ad 7c 7h| 57.38%
This is because having the nuts for low is more important than having the nuts for high when there are three low cards on the board. You have five outs to a scoop (three aces & two deuces), while your opponent has seven outs on the flop (and three more on the the turn usually) to scoop you.
Its not like you can get away from this hand, but since you're almost always behind for low, sometimes it can be prudent to take a card off.