Many cake recipes make 3 layers but my oven only holds 2 on one shelf. Can 2 shelves be used or can batter sit while the first ones coos coo

Don't let the batter in a 3rd pan wait while other 2 bake...the chemical reactions of liquids, b powder and/or b soda will exhaust themselves and deflate at room temp. Rather, put all 3 to bake at same time and rotate if needed to get even heat.