Opening is a specialist position. It's a lot harder to face new ball bowlers at Test level, than at domestic level. Hales is an opener (or will end up one) so by putting him at 3 he has an easier ride of it, if the openers see off the new ball. Taylor meanwhile, may bat at 3 a lot, but he does that behind players who see off the county new ball more often than not, so putting him into a Test where he could face the first over, against someone like Steyn doesn't make much sense. Just look at Root, he looks great at 4, yet struggled opening, because it's a much harder job.
Plenty of openers have started life at number 3, because it's easier. Much like plenty of quality Number 3s have started life lower down the order for the same reasons. It's a lot easier to move players down the order (which you'd be doing in my suggestion) than doing the opposite (like moving Ballance up to open say). With these two, they're new to Test cricket, so why not make life easier for them at the start and then move them up after they start doing well?
