Rumor is that it was in development since shortly before Res 2 Remake released. Supposedly Capcom themselves aren't making this one, they contracted it out to a 3rd party studio. It will make use of Res 2 Remakes engine and reuse some Res 2 Remake assets to help speed up development as well.
I find it hard to believe Capcom would let anyone else do RE3, so I think that may be an assumption based on the fact that they're letting Neobards work on Project Resistance.
I know they'll re-use assets from RE2. That goes without saying, but that's still a very quick development cycle for such a high quality game, even considering that.
Well, they could be a bit busy with Res 8, especially if they are upgrading their engine with next gen tech for Res 8. So I wouldn't be surprised if they at least contracted out part of the work on Res 3 Remake to a 3rd party studio.