If we're going with 2 offensive bots and 1 defensive bot, it would have to be 971, 148, and 4607.
971 and 148 are both lightning fast shooters, have flawless autonomous and are nearly unblockable. They both also have really good drive teams and super consistent and fast climbers. 971 also has the most consistent 2 ball auto I've seen. These two robots alone could easily score 20+ boulders in the high goal.
On the defensive side, 4607 would be the best choice. I may be a little bias here, but 4607 was easily the best defensive bot this year. They pretty much shut down 5172 in this
match. Their robot is fast, robust, and has strong pushing power. Their drivers know the best way to play defense. They know where to go, when to do it, and how to do it. 4607 also can score a few boulders in the low goal, and they have a consistent climber.
I'm pretty sure this alliance would beat out any other potential "dream team", even those with 3 offensive bots. Plus it's also the only "dream team" alliance that actually realistically could have existed. I considered putting 5172, 254, and 2056 in over 148. But 148 has a better climber than 2056, more robust and consistent than 254, and can handle defense better than 5172.