It should be fine as long as you use separate thermostats for all 3 vivs. I would house the royal at the top though as a ceramic could make the other vivs too hot. No idea if this could be an issue for the beardie as I don't keep any so no idea what heat source they need. I have a double stack and only had temperature problems when the stat broke and I used one stat for both vivs until I got a replacement.
Corn at the bottom. The other 2 both need lamps/ceramics which will cause a very warm area in the viv above due to being mounted to roof. Beardie in the middle as there temps need to drop at night which could be an issue if they were above the royal. Also make sure the warm ends are same end.
You could also ask ya friend to build it so there is a void between then with vent holes to allow air flow that would stop heat transfer between vivs a little bit