3 bet 56s and 78s from late position with several limpers or just call, with 100 BB's ?

Presuming we have little-no info on opponents. And presuming it is a low-level game (as limping is not seen in higher stakes).
I would use a mix strategy, leaning towards flat-calling, and sometimes raising.

I think it's good to build the pot when in position with a hand that hit-or-misses the flop.
However keeping the pot small by just flat-calling will reduce variance.
From my experience players will be willing to call a lot with top pair when you hit 2pair, and also call a lot with 2pair when you hit your straight.
But hitting the flush you have to be cautious either way (by 3betting or calling) as your flush is quite weak.

Either way, I would not 3bet large as those hands play better multi-way than heads-up. And, as mentioned, you want someone in the pot spiking a good hand when you have 2pair or better.

That's how I see it anyway
Hope that helps

As i play at low limits, i like to flirt with 56s and 78s in some spots .. before, i didn't bet much with these hands, however nowadays i see that it is possible to bet or just call depending on the situation of the tournament or table and win some good pots ..