You can get an Amplifi base unit for $275 and then add repeaters at a later date.  It's just that doing it that way you'll pay about $100 more in the end than if you bought a 3 pack up-front.  But that's what I did, got the base unit first for a while and then added a repeater as required.  You might find that the base unit gives you better Wi-Fi than you have right now anyway, given the antenna setup inside the unit is quite unique.  It covered a lot of areas my Airport Extreme had trouble getting to.