I ride sportbikes as well as my Fatbob. I also ride a friends BMW touring bike sometimes when he's away on business. I just adapt to the bike I'm currently on and I'm fine. The Thunderbird feels VERY similar to my Fatbob, the R3 is taller with a more upright riding position. I'd probably want the more forward, Tourer floorboards and controls.
All that said, I REALLY thought I had my Fatbob would be sold over the weekend but it didn't
So right now all is a very moot point