Hopefully the Sinister Six movie is an elaborate lead-in to a massive all out war with Spidey in ASM3.
Why would it need to be?
You could very easily make Spider-Man's battle with the Six the entire focal point of their titular film - while also resolving the lingering issues from the end of TASM2 as far as Peter Parker/Spider-Man is concerned, and then tell a completely different story in TASM3.
Garfield is still only contracted for three Spider-man films. Don't expect to see him in Sinister Six, maybe just a masked Spider-man.