Has anyone used helicals in a foundation? Can they be used in a pier and beam foundation that would raise the first floor about 36" off grade?

I have. The porch corner slab was supported by stem walls and spread footing. Customer generated a request to switch from framing and stucco arches (2), to solid concrete arches so the engineer specified the helical (1) below the corner to pickup the extra load. No complaints after the fact, fifteen years later.

Contact your local installer and ask them. Also ask your engineer.

Hughianowen,
A quick Google of "Helical Piers" shows a TON of info!!!