3 different Notices of Furnishing, or one?

It all depends on the notice of commencement filed by the GC (if the GC filed a notice of commencement at all). If the GC filed one that encompasses your three different contracts, then one notice of furnishing by you would suffice. But if the GC filed separate notices of commencement for each portion of the work, then you would need to file three notices of furnishing. 

If you don't know how many notices of commencement were filed, I would go the "better safe than sorry" route and serve three different notices of furnishing. That all being said, I don't see a problem with sending the notices in the same envelope.