15 hours this summer?

Never mind, I didn't read it carefully.

It depends on how much they overlap. If they totally overlap that would be the equivelent of 30 credits as far as work load. If it is just a week or two, you could probably do it if you don't have much else going on.

no overlapping, session 2 ends then session 3 begins a couple days later.

If you can stick to a good study schedule I think it is doable. Last summer I was scheduled to take 13 credit hours but dropped one class about a week before the semester started because everyone told me not to do that. How I wish I did not listen to them. The classes I was taking was Intermediate Algebra, Intrerpersonal Communication and Psychology. I so could have taken that extra class without failing anything. I was under challenged. I also have two little ones and trust me I still had plenty of time to dedicate to them. So I say if you have that discipline go for it.