In my opinion, you're probably going to be okay if you span 3' with one 2"x12", however, you may consider putting at least one of those 2x4's beneath it, skinny side up. If you can fasten it very well to the bottom of the 2x12, you shouldn't see any bowing. If you think it may still bow, put two 2x4's under your 2x12, thin side up. As far as legs go, if you want to build them yourself instead of with cinder blocks, build up the sections. Stick your 2x4's together to make some composite 4x4's and use those as legs. If you build it strong, then make it a little stronger than you think you need, you'll be fine. Just don't forget to throw a few cross braces in the mix (if you don't use blocks). Technically, you could make your cross braces with cables! Just make sure you use 2 cables, top right to bottom left, top left to bottom right!