I might be wrong but 3" FT could induce some vignetting with large chips as the ones you mentioned?
Maybe at least the true 3" FT then? Also, Precise Parts can machine pretty much any adapter you need. Costly but good quality.
Any particular reason why you want 12" RC for imaging?
Downsizing the scope and camera pixels can make life easier and save you $$$. To my knowledge, with ASI 1600MM you only need about 800mm fl to reach the limit of what can be captured with typical Australian skies with long DSO exposures (not Solar/Moon/planetary).
A very good option IMO at the moment is the 16200 chip.
Anyway, just sharing some ideas