Some interesting analysis from very good posters, but I think it's a little more straightforward than some are making it out to be:

1. Solid SB raises several limpers preflop. Range: Big pair, AK-AJs, some suited connectors or big suited one-gappers.

2. Solid SB calls a flop 3 bet. Range: We can eliminate the suiteds unless they're spades. So now we've got AK, KQs-KJs, or two spades.

3. We still have girl to worry about.

So, when the third spade hits the board it seems exceedingly unlikely that you're ahead of SB, with a good chance that you're drawing dead. Seems like a pretty clear check to me, and possibly a river fold for even just one more bet if it comes from the SB.