Yeah it MAY be possible (because I don't think Blizzard expects there to be coordination from all 5 people in a lvl 17 instance; Some kid is always sitting there dicking around.) but filling the other two slots would probably make things WAY more interesting. You would definitely not be able to 3-man instances like ramparts (at the standard level for that instance, that is.) so you may as well start playing with Pugs (Or friends with levels close to you that will understand that you'll be faulty) to get some practice with coordinating all 3 of your characters. Since you'll have to do it eventually you may as well get used to it now, IMO.