The obvious parallel to the James-Wade-Bosh superteam assembling in Miami is the Wilt-West-Baylor Lakers of 1969-1972. That triumvirate was only around for one year, 1969, as Wilt missed almost all of 1970 with an injury, and Baylor played a combined 11 games in 1971 and 1972 before retiring. The main difference between those two teams is age-all three players were in their thirties by the time the team was put together, which led to declining performance and injuries, a fate that doesn't seem likely to happen to these Heat (unless Wade falls apart due to injuries).
The short answer to the question, "Has this happened before?" is no. However, there is one team from the NBA's prehistoric era that bears a passing resemblance to these Heat; the 1956 and 1957 Philadelphia Warriors. In those two seasons, the Warriors had the NBA's two best offensive players, Neil Johnston and Paul Arizin, little depth, and weak interior defense. In 1956, the Warriors rode the league's best offense* to the NBA's best record (45-27). They fought past the scrappy Syracuse Nats** in the Eastern Finals, and then blew past Fort Wayne in the Finals. The Warriors fell back to 37-35 the next year with the loss of Tom Gola (the team's third best player), and the arrival of Bill Russell in Boston, making the division quite a bit tougher. They fell to the Nats in the first round of the playoffs, and Neil Johnston would decline the next year, ending the team's "run".
All of which is to say, a team with two superstars and little interior defense has won a title, but the last year it happened was Bill Russell's senior year at the University off San Francisco***. I'm not counting the Heat out (the additions of Haslem, Miller, and Z make that roster even more formidable), but that fact has to encourage the Lakers.
*The Celtics scored more points per game, but that was a function of tempo (read: Cousy). The Warriors were much more efficient.
**This was an interesting team in the mid-50s. They had one superstar and a bunch of scrappy guys (Paul Seymour, Earl Lloyd, George King) who couldn't shoot, but played defense and won a surprising amount of the time.
***With the possible exception of the Lakers during the last couple Showtime championships (on the defensive front, they were much deeper than this Heat team).