Right- all you need is a three gallon carboy (or three one gallons, but that's a pain) and you'll be all set. You don't want to top up 2 gallons! Plus, then you'll have to rack, because it'll restart fermentation, and lose 1/2 gallon so you'll have to top up, etc.
I always make extra must and just use a growler or wine bottle or something to hold it and then use it for topping up each time I rack.
The first time, I'll go from a 6 gallon batch in a bucket to a 5 gallon batch in a carboy with maybe a wine bottle left over. You really lose alot to lees in fruit ones due to the pectin and solds.