I have three loans serviced by the U.S. department of education. The servicer, Great Lakes, has provided three individual 1098-E forms for these, should I list them separately or add them together in one entry?

It does not matter.  The entries will get entered onto your tax return as a total either way, so its up to your preference as to how you enter them in TurboTax.  Its easiest to enter them as totals per lender.  Personally, I enter my separately so I know every year to expect X number of forms, so I don't miss any forms.