The Celtics had Paul Pierce, Ray Allen, and Kevin Garnett. Out of those three, only Garnett was considered a certain hall of famer and a top 5 player in the league. At the time, Pierce and Allen were not considered hall-of-famers, now they're either "certain" or "borderline" depending on who you ask. They were both probably top 15-20 players at the time.
The Heat have LeBron James, Dwayne Wade, and Chris Bosh. LeBron and Wade are almost certain hall-of-famers at this point and considered top 5 players, perhaps top 3 players in the league. LeBron is considered perhaps the best player in the league. Even if you think Bosh is overrated he's a top 5 big man in the league going forward.
I don't think there's a comparison here in terms of the level of talent that is being bunched up on one team. You could probably compare it to other big threes in history, though.