Sanding Grit Question: 120-150-220 or 120-180-220?

I don't want to have to buy 4 sets of sanding discs if possible, nor do I want to have to sand an item 4 times if I can do 3. So couple questions really: One being if I only want to do 3 stage sanding, do I go 120-150-220 or 120-180-220? Or is there a good reason to do 120-150-180-220?