Welcome to MySwag, my guess with three kids would be a soft floor, it also depends on your budget. If you are set on a hard floor I would be looking for something with a kids room which is attached to the end or side so for those longer stays you have a bit of room.
The beauty of the soft floor is a bit more room, especially if you get a walk up model. I have two kids and we have a good amount of room when the kids are in the trailer due to the weather turning bad, most of the time they are out playing anyway but it is great when needed.
Good luck with your search and ask away if you have any more queries, and also keep your eye on the classifieds, bargains do appear
GG