This could also be another scenario:
Wade and Bron opt out. Resign with Miami and take a bit less so Melo joins them.
Bosh opts out and agrees to be signed and traded to Houston for Asik and Lin.
Possible?
Not if Lin and Asik are going to Miami, unless Wade, Melo, and Bron take a lot less. The new big 3 would have about $40 mil to split amongst the three of them if there is no sign-and-trade with NY to bring over Melo, assuming their roster is the new 3, Asik, Lin, Cole, their 1st this year, and minimum salaries, and they've renounced everyone else.
Okay...so another words, highly unlikely?
Because the more I think about it, I think something like that could end up happening.
Lebron and Wade stay in Miami, bring Melo in, Bosh goes to another contending team (Houston, Dallas, maybe the Lakers.)
The Heat then use their MLE on a center
Bosh would have to go somewhere where he'd just sign via cap room. The Heat simply don't have the cap space for Wade, Melo, and LeBron unless the cuts are very significant. I forgot that Haslem has a $4.6 million player option. Apparently he's undecided on what to do, but if Joel Anthony is going to take $3.8 million to stay in Boston, I'd bet Haslem stays in Miami. If they're letting Bosh walk (so not a sign-and-trade), then Miami would have about $17 million each for their new Big 3, which is more reasonable than the $13 million they'd have if they took back salary in a Bosh sign-and-trade.
Bosh could still go to Houston. It would just take a third team to absorb Asik and Lin. This is why the thought of Boston taking on both for free, or even being compensated, make sense, because a three-way trade makes sense for most of Houston's free agent aspirations.