Good question.
I don't think that there can be any argument that Willie Miller would have to be one of the three. He was the focal point and driving force of our best team and is easily the most influential player we have ever had.
For the other two, it's pretty hard and there are so many good arguments for so many good signings, but the two I'm going to go for are:
Theo Snelders - In my wildest dreams, I never thought we'd replace Jim Leighton so adequately. A wonderful goalkeeper who stepped straight into a great relationship with Miller and McLeish in particular.
Charlie Nicholas - Came in at a time when there seemed to be a bit of a cloud over Pittodrie; We'd lost Fergie, the team was breaking up and generally not doing too well, Porterfield was relying too much on just having a good defence to win matches meaning we were being far from entertaining. The whole club needed a lift, in particular the players and the fans and Charlie Nick is probably the most glamorous signing we will ever, ever have. It lifted everyone and I've never seen so many people so excited by a signing at Aberdeen. Proved to be a great player for us and one of the best I've seen in the red shirt.
Have to say that Frank McDougall was close for me too. Replacing Mark McGhee was a huge ask, and Frankie did it brilliantly. Just ashame we only got two years out of him because of injury.