You don't mention the ages of the kids. If they are all under 10, then a crew cab should be fine. But if they are over 10 years, you will be facing a very long drive with three teens stuck side by side. In that case, I agree with JRscooby to consider a van. It would give more room for the kids and their stuff to keep them happy on the drive.
Depending on the ages, a tent or two could allow for a smaller trailer thn otherwise. A hybrid trailer could provide extra beds without having to increase the trailer size.
If looking at bunks, watch the gvwr of the trailer - some of the bunk layouts use up most all the gvwr on lighter trailers. And since you will be boondocking, you will want to be able to carry a full fresh water tank plus all your gear. Also remember that the west is in a drought - being able to fill your fresh tank just anywhere is not likely, so you'll want to intersperse a full hookup site here and there so you can easily refill and dump tanks, as well as charge up the batteries.