1. You don´t give 90 minutes to three players. No NBA rotation works that way.
Do you mean Kevin McHale never got 30+mpg coming off the bench behind a forward and a center who were also playing 30+mpg? (Okay, so I admit that's partly because Larry Bird could slide between both forward spots.) Another example is Roy Tarpley getting sixth man of the year behind James Donaldson and Sam Perkins. There are probably more examples with combo guards. The main reason you don't split 90 minutes between three players is that you usually don't have three players good enough to play 30+ minutes per game, not because it doesn't work.
Does Kevin Garnett still possess the agility and speed so that Doc can occasionally throw out a Garnett/Wallace/Perkins big lineup out there to create a different look with matchup problems for some teams? That could help get some additional rest for Paul Pierce, especially in the playoffs.
And regardless of whether we do a 3 or 4 man rotation during the regular season, we absolutely do a 3 man 4/5 rotation come playoff time. So again, while Davis would help in the case of injury or foul trouble, he really wouldn't get much run in the playoffs. And being that this team is good enough to win 65 games with Mikki Moore as the 4th big man if it came down to it, I don't think it would be catastrophic if Baby walked and we went with someone like Skinner, Ben Wallace, or even Scal as the 4th big.