Out to those that left the bolt out it's there for a reason why would you not put it back in? Would you leave a head bolt, intake bolt, exhaust bolt, or any other bolt out just because it's a pita to get too? I would think not so take the time to do it right or don't do it at all!
most cars only run 2 bolts from the factory, for some reason they decided to go 3 bolt on the mustang. 2 bolts will hold that starter in just fine...its not like its resting on the bolts, its rests in a pocket and the bolts secure it to that pocket.