Well, remember that I'm talking 4th quarter here; the last 6 minutes. All 3 (Wade, James, Bosh) will undoubtedly be on the floor at the same time if the game is close (barring fould trouble).
I think that this might not happen, though.
From a PR standpoint, you have to have your superstars on the court at the end, but in most cases, in a close game, you want your best defenders in the game, and then a go-to scorer. I think that Lebron and Wade will stay in to take shots and you might be better off to have a better defender in the game than Bosh.
In a normal team circumstance, you're right, on a team where you need Bosh's scoring ability down the stretch, but on the new HEAT, I don't think you do.