Most important buildings in 3 player version
This is an interesting question in part because I think it will depend on what your opponents have. Blue buildings trigger on every roll, but don't tend to generate as much income as the green buildings. If your opponents have a lot of red buildings, I'd guess it would be more helpful to have green buildings so that you can earn a lot in one go (and red buildings trigger first). Blue buildings can have income trickle in more regularly, but the income will trickle out for your rolls that trigger red buildings and for their rolls that trigger purple buildings.

Red buildings will probably still be useful, though not as useful as in 4 player games.

I have not played a lot of three player games of Machi Koro, but I would guess that it provides the best balance between the power of the differently colored buildings.