I think if you're planning to start with 3, start with 3 bantams first, and see how it goes from there. I don't have any bantams myself but I'd think it'd be easier to add large chicks to small adults, vs super tiny chicks to much larger adults.
But it's up to you really, if you're not completely stuck on the numbers, maybe by watching how your first group does in the coop and run, you'll have a better idea of how manageable it'll be to add more birds to that space, regardless of bird size.
But it's up to you really, if you're not completely stuck on the numbers, maybe by watching how your first group does in the coop and run, you'll have a better idea of how manageable it'll be to add more birds to that space, regardless of bird size.