No! Absolutely not. Walmart abuses their fish, and anyone who buys fish from walmart is contributing to the horrible fish abuse. Every time I go to Walmart, their tanks are plagued with ich, and dead fish will sit on the bottoms of the tanks for weeks before they are removed. Do not even get me started with the betta and goldfish abuse at Walmart! Never buy fish there!
First, make sure your fish tanks are cycled. You can read up on fishless cycling on this forum or on google. A betta is alright for the 3 gallon, but I would recommend an air bubbler since that is a very small tank. For the ten gallon, you could place a solid tank divider in there and have one betta on each side. Or you could keep a pair of dwarf gouramis in there. But, no matter what, I strongly recommend that you do not buy fish from walmart. It seems as if you are somewhat new to the fish hobby, and you probably would not know what to look for as far as illnesses in the tanks.