I have to disagree with the OP. First let me say that we just need a star period, position doesn't matter as much as getting one. That said, if I could get an equal talent at any position I think the 3 would be the last one I would take. The OP thinks it is our weakest spot, I think it may be our strongest spot. Crowder is already very good on D and is showing to not be a liability on O. He will only get better too.
So, if I am Ainge I think my first goal is to bring in a star 4 or 5 or a big 3 that can play the 4 a la PP the last few years. Our D is what is making us win. We've got something very special with our 1 2 and 3 right now.
But as I already said, I'd take a star at any spot because they are so hard to get.