I don't see why they'd need Thor right before A3, after all, we just had GotG and that has nothing to do with Age of Ultron.
Also, that would be a full 4 years after TDW, I don't see them wanting to leave it that long. May 2017 is still 3.5 years after TDW, but it's the nearest available with the Doc taking July 2016.
That wouldn't have to happen though, as I said above, we won't be seeing the Guardians in Age of Ultron despite the former being the last film released before the latter.
I don't read the comics so I don't really know anything about the Inhumans (As much as I knew about GotG when that was announced). I know more about BP & Cpt Marvel because of hanging out here and the cartoons, so I might lean in their favour purely for that reason. Feige has talked more about Inhumans, although he has also recently said he keeps getting asked about BP & CM:
http://screenrant.com/black-panther-captain-marvel-demand/
"“They’re both characters that we like, that development work has been done on and continues to be done on and certainly the public… it’s a question I get asked more than anything else. More than Iron Man 4. More than The Avengers 3. And that’s sort of the fist time that’s really happened to us so I think that makes a difference. I think that’s something we have to pay attention to.”
“I’ve always said we have our plan and it served us very well and there hasn’t been anything that has made us deviate or change from that based on opinion because opinions online change and they alter, and they’re based on sort of limited information they have towards what’s going on behind-the-scenes. But in this particular case I think it is a groundswell. I think it does. I think it means something substantial.”"