Would a slotted flap still work if the prop is located mid wing between the wing and flap?
Yes.  Inserting a prop aft of the wing and forward of the flap would not reduce the flap's effectiveness.  If anything, the stronger, more concentrated propwash would make the flap more effective.
The answer doesn't change for a triplane.  But off topic, I wonder about how to design a triplane that's light enough to be in the ultralight category, what advantages it would have over a biplane or monoplane.  Commercial interest in triplanes petered out eighty years ago.