Well you will need something around 4 gallons of wort to boil for one hour down to 3 gallons. You could calculate how much water to use in the beginning and how much the grain will absorb..but I say it would be easier to just use a simple qts/lb ratio. (Typically 1.25-2 qts H2O / lb of grain). After you pull out the grains and drain them, either top off to 4 gallons, or put a strainer over the brew kettle, your grain bag on the strainer, and pour your top off water through the grains to give them a little rinse til you get to 4 gallons.
Edit: The nice thing about doing it this way is that is easy to use c1v1=c2v2 to help you judge how much volume you need to hit your target gravity, if you are concerned about it. If you add all of your water first and you find out that you used too much, then you pretty much are going to have to boil for a longer time, or you will end up with a weaker beer.