What's your primary concern here? Boil size or grain bill? Not quite sure what you're going for, so not sure which way to point you...
I mean, if you're trying to mash with one gallon of water, you could easily mash 2 or 3lbs of grain in that volume, then sparge with few quarts or what have you (the full 2 gallons may be overkill), then top off your volume to 3 gallons and bring it up to a boil from there.
If you were dead set on mashing just 1lb (you're really not going to get a whole lot of fermentables or enzymes from just 1lb, but we'll go with it for purposes of the example), you could still do so with 1/2 gallon of water, sparge with another couple quarts, and still then top that off with enough water to reach your 3 gallon boil.
But it'd really help to know what you're trying to get out of this thought experiment.