Please no.
If Marvel is going to have a Spider-Man in their Avengers, then it should be someone who is at least in the correct age bracket. Garfield is 30 right now. By the time they release ASM 3 in 2018, he will be 34 years old.
Spider-Man works best as a teenager who becomes a 20-something. Both Tobey Maguire and Andrew Garfield were vastly too old at the outset of their respective endeavors and really aged themselves out of the role by the end of their runs. If I were Sony, I would find someone who would be 25-27 by the time three films have been completed, that way you can still milk one more movie out if you cannot find a good replacement in time.