For a 72" tank, 2 72" bulbs and 1 48" bulb is the max you can do with a single ballast. 48" bulb would go in the back since the overflows are in the corners you don't need a full 72" anyway.
What I would do is 1 72" UVL brand Actinic/White bulb, 1 72" SuperActinic Bulb and 1 48" Actinic/White bulb.
I just gave away those bulbs that I was using on the tank but do have a 48" Actinic/White and a 48" AquaSun bulb that you can have still have a good 6-8 months use left on them.
Dirk Griffin is best to get the bulbs from. He is local ( RedWing but makes trips to the cities often ), so you don't need shipping which can be pricey for 72" bulbs.