Well a bonsai book is a good place to start. As forums aren't always an instant information machine. My biggest recommendation is to be more patient, as it is a rather important skill in bonsai, and maybe the hardest to master.
But I would not cut any of those three shoots at all. Three branches coming out of the same exact spot can produce reverse taper, a bulge of sorts, but it looks pretty clearly that one of them is a bit lower than the other two. Furthermore the tree itself is quite small and has little foliage as is, instead of think about what to chop off I think you should let it grow freely. Preferably in the ground for a few years so it can put on some trunk girth.
Also, you may want to look into how the leaf size will reduce on bur oaks. I know that leaves on mature trees are quite large, and with a trunk that small this may not be the best possible candidate for bonsai. At least t the moment... stick it back in the ground and it'll beef up. Research other species you may want to work with that will do well in your area and have a small leaf size that lends itself easily to bonsai.
