I'm looking to replace my old orbit valves with 3 of these, want to place them underground, but what's the best sized box to buy to do that?

Your valves should be already in a valve box, so you can just replace the orbit valve with the Rain Bird. If you want all valves to be in one valve box, the Rain Bird standard rectangular box will accommodate all three valves. Its dimensions are; 14.5" L X 9.4" W X 12.1" H. Hope this helps!