Offsite resort guests are going to get access to Fastpass+ as well, as it is included with admission (this was stated early on). In fact, tomorrow at AK starts a full-blown test - NO regular Fastpass, Fastpass+ only.
There is no capacity to offer everyone at least three, and then increase that for higher tiers of guests.
Everything points to 3, and that's it. Occasionally, if the capacity vs. crowds allow it, you might get an extra.
Whether offsite guests will get to book in advance remains to be seen. Disney has in various ways implied it, but for tomorrows test they'll be doing it at kiosks only.
What we're starting to notice is that onsite guests get to pick FPs starting at 60 days from check-in - through the length of their reservation. So even if offsite guests get 60 days, onsite guests will get a "+" benefit similar to with ADRs.
In the long run, if they see FP+ being underutilized, they might offer more, either opportunistically, as a "bonus" for certain things, or just a flat one or two extra. It remains to be seen how this pans out. Knowing that there were guests out there that just "didn't get" Fastpass, I can't imagine how this will be simpler for them.