Several points. First,
DVC does not deposit the 3 BR units and even if you got a trade through II, it would only be for one unit unless you could lockoff and deposit as 2 units with II. That assumes that the unit is even with II, many in Destin are only RCI and I think the only Panhandle II resort with 3 BR is Landmark in Panama City. The only RCI 3 BR I could find in Destin was Landmark Phase II but the lists are not always 100% accurate. Or in some cases the owners looks at it as a 3 BR but the exchange company only lists it as a 2 BR.
I would doubt one would find any interest from a DVC member trading a 3 BR GV or a 2 BR plus studio for this unit. The truth is that even if it were 4th of July, the comparative value is simply not there. Now if you were willing to take 5 days avoiding Fri and Sat nights, that MIGHT and I repeat might, get some interest. Even then I think it's doubtful for the unit size(s) you quote. The most I think you could expect would be a 1 BR for 7 nights or a 2 BR for 5 nights avoiding weekends. And that assumes a mid summer week, anything that's at all off time would be worth even less in terms of DVC points.
I would think they're best bet would be to rent out what they have then rent from a DVC member. Even then, getting a 3 BR will be difficult at best. IF the resort is with II, they should in a trade request for 6 people for the dates desired. II will usually require a combo of 3 choices. Maybe 3 resorts or 3 weeks. Just list all the DVC resorts. If they are with RCI, they should trade for off site or rent what they have and pay cash. They should also list on the Rent/Trade board and hope they find someone that needs exactly what and when they have to get the most value exchanging.