We have 3 kids also. It is tough to fit 3 in our 21' Escape without using the floor if your kids are big enough you can't fit them all on the dinette. Our kids are 5'8, 6'3", and 6'4" and two is already tight on the dinette. Our standard configuration is to bring a tent. If two want to sleep inside (hot weather or...), they take the back board of the U dinette and put it between them. Our daughter has thoroughly warned her (much larger) brother that if she gets pushed out of bed, he is sleeping in the tent the next night
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This has worked well especially since we have had numerous international students stay with us over the years and the tent provides a plenty of room. As the kids got older they liked having their own space.
The problem is the occasional campground (~10% of places we stay) that doesn't allow a tent with a trailer. Our son usually chose to sleep in the truck rather than the floor of the camper (F-150 crew cab). However, he got a job out of state a couple of years ago and now only camps occasionally with us.
A homemade bunk above the dinette always seemed feasible to me, but the above situation always works well enough it never moved up the priority list.
I hope that helps a little. Enjoy the camping and all the great memories!