You can do any recipe, any size with biab. Since you're doing all grain you can just order the grains separately to do 3 gallons instead of the usual 5 that kits come in. I've had a lot of luck with the recipe database here to pick my recipes. I think brew masters warehouse has a feature where you can build a recipe and just order the grain you need, but I've never used it. As far as scaling the recipe down, download brewing software and that will do all the work for you. I personally use beersmith and know they have a free trial but the are others out there as well. Good luck, if you have any other questions just shout.