Narli:
Don't second-guess yourself.
Yes, you might get one of the trundles that are more like a thick school-nap mattress than the plusher half-mattresses (they vary from room to room), but with three boys, I think you'll find, like our family did, that the kids will be fighting over who gets to "sleep on the floor." Although I don't recommend it for your 10-year-old, your 7- and 5-year-olds will be fine.
Unfortunately, your other option is to upgrade to a Deluxe resort which sleeps 5 with its daybeds, a cabin or family suite that accommodates 6, or two rooms at the Value resorts. For the money, I just don't think you need to upgrade with the ages of your boys. (I can think of many other things to do with the money, like a
Pirates & Pals cruise, dinner show, or some pretty awesome souvenirs for that age!)
Wishing y'all a magical vacation!
Whitney