When I do batch I just use the amount of water in my recipe and figure in some extr for loss to being absorbed into th e grain bed.
I split the water 50/25/25 approximaely. I've measued the three for specific gravity just out of curiosity, but I mixed all three of the pots back together to gain consistency.
I recommend just make sure you use enough water for your recipe.
I didn't use enough water one time. See my link below. This some pretty good beer.