While 2 of those 3 have already moved from 3B. If you specifically want a 3B, the answer is Alvarez. It doesn't seem very likely he stays at 3B his entire career though. Take out position an Alvarez is the best prospect anyways.
Out of the 3 the order id put them in is Alvarez, Wallace, Viciedo. As for Moustakas and Vitters added in...id go with Mous (long term, less likely to succeed), Alvarez, Wallace, Vitters, Viciedo.
A little info on the guys who are available now:
Alvarez: Going to be a power hitting 3b. He will not hit for average at the MLB level. Likely to have Adam Dunn like HR and AVG. Something probably around a .260 avg and 30+ HR. Alvarez long term could become a 40+ HR guy.
Wallace: He is going to be a well rounded hitter. He probably wont be much of a big time power guy likely sitting around 25 HR but should hit for a nice average at or around .300
Viciedo: Still raw compared to the other two, but is supposed to have great power. Has poor discipline at the plate and has a lot of things to work on. My ceiling would be 30 HR, .270 type hitter. My realistic guess is that if he remains a big power guy his average will suffer and will be around what i said above for Alvarez. Id give Alvarez a way better chance at succeeding though. Dont see viciedo as a 40 HR guy ever.