It's probably too late for that now. The time to figure out if he could handle 3B was in the minors. I'd really be leery of taking a guy who's a cornerstone of your offense and asking him to learn a completely new infield position in spring training.
I definitely think that if the Braves are planning to extend McCann's contract, then they need to be thinking about getting him out from behind the plate some. Brian doesn't strike me as someone who's going to use all the, uh, enhanced techniques that let Ivan Rodriguez catch forever. Maybe Freeman could learn to play a little LF and McCann could play 1B 20 or so games a year. Something to save the wear and tear a little bit.
There may be a perfectly good explanation why the Braves never tried Freddie anywhere else. He's just so good with the glove that I'd like to know what it is.