Welcome aboard, Derek.
The three stacked two by fours works fine. You will cover them with either sheetrock or backer board. Then the Kerdi. I also make curbs out of single two by fours laid on edge and covered with drywall. Makes for a narrow curb that I think is more pleasing to the eye. Don't do this last one over a concrete floor, though. Use bricks for that and cover them with Kerdi.