As a basketball fan, I can buy your claim that he could have won 3 NBA championships, but I think two of them would have been in the eighties when the Celtics still had Bird, McHale and Parish playing at a very high level. Then again, the Lakers were still an extremely awesome team and still had enough talent to beat a Celtic team that had Bias on it in 87 to 89.
More likely, Bias' presence on the Celtics mitigates the rise of the Pistons and they remain a contender, but certainly are not in a position to win NBA titles in 1989 and 90.
However, you also have to factor in the rise of Jordan's Bulls into the championship picture. At the very least, I don't think he'll win 6 MVP awards. That's just not possible when he's playing in the same league as Michael Jordan, who in the 1990s would be entering his prime, nor would I think he have 5 scoring titles, when again he's playing at the same time as Jordan in his prime. Also, the league tends to hand out MVP's to the primary player of teams that do surprisingly well in a particular season, like Charles Barkley's MVP award in 1993 where Pheonix went to the Finals, Karl Malone's MVP in 1997 when he led the Jazz to the Finals, things of that nature.
Also, due to butterflies, maybe Magic doesn't get HIV and continues playing straight until the mid-90s, and the Lakers remain a championship contending team well into that time. Magic led the Lakers to the Finals in 91, and with his abrupt retirement due to HIV the Lakers stank (relatively speaking) until the Shaq-Kobe combo (again they would probably butterflied away in this scenario). The Lakers have been a historically good franchise and Bias playing out his career is unlikely to affect that in any drastic way. They'll find ways to get good players and its likely they would find another franchise player after Magic retires given their historical trends. (Of course, that likely means that Shaq and Kobe aren't coming to the Lakers, but they'll probably still find someone else that was born in the early to mid seventies to be their franchise player into the 2000s)
I do fully believe that Bias was a franchise level player and that he would have likely stayed in Boston his entire career. If Bias were to have five scoring titles in the same era as Jordan, there must have been a particular stretch where Boston couldn't assemble a good supporting cast around him (say in the mid 90s after Bird and co retried) and was forced to scroe in huge quantities in order to keep his team competitive.
My supporting evidence is that Kobe averaged like 36 points per game in 05-06 and had a stretch the following year where he scored 50+ in four straight games, but the Lakers were relatively bad in those years, and he was the only viable scoring threat. Despite these feats he did not receive an MVP award, and Bias in a similar situation would not get MVP's either.
As for Jordan being able to get scoring titles while still winning multiple championships, Jordan was on an entirely different level than any other basketball player ever to live.
As for Bias' last championship. It would probably come at the twilight of his career after Jordan retires. They would have likely had to build a new supporting cast with Bias having a secondary role and not being the franchise player anymore. Who these players around Bias are anyone's guess, with a POD that's over a decade now, but they certainly aren't Paul Pierce, Antoine Walker, Reggie Lewis (another Celtic that met a tragic end) or any other OTL Celtics.