According to beersmith (using the water needed calculator) the grain will absorb app. 1.86gal, so you will need:
Water into BK: = 5*1.15 = ~6gals
Wort drained from mash = 5-1.86 = 3.14gals
Sparge Water needed= 6 - 3.14 = 2.86gal
3gals seam realistic....
Are you batch or fly sparging? If you batch sparge I would split the 3 gals in two 1.5gal cycles. I highly recommend Bobby_M's NMODBS method found here (essential AG beginners reading) :