Here is a nice scouting report on Brett Wallace's defense at 3B.
Brett Wallace defense scouting report | Project Prospect
That seems to pretty much be the consensus. We'll see what happens, but hopefully he can improve and stick...or at least give the A's a few years there.Conclusion
If there's a way that Brett Wallace can stick at third base, Brett Wallace will make it happen.
It's very unlikely that he'll be a career third baseman in the big leagues, but he wouldn't be a liability at the position if the A's called him up today. I could see him spending a couple seasons at the hot corner, provided that the A's are willing to take a bit of a defensive hit. Expect him to spend the bulk of his career at first base or designated hitter.
There's a video at the end of the article. His throwing motion looks like he's short-arming the ball. Perhaps it was just because he was focusing on fielding the ball and lolly-popping the ball over there. I don't know.