Do these two necessarily have to be the final boil volume, or is it acceptable to add water to achieve the desired boil volume? I am planning on doing a few experimental 2.5 gallon batches, so I want to boil about 3 gallons.
No, they don’t have to be the final boil volume. Especially if they are partial mashes.
I’d say use about 1.5 qt/lb for mashing and the same amount for sparging. If you have too much sparge water you risk oversparging which can lead to excessive tannin extraction.