I know this is years old. But Ive been using 3D text for years and never had this issue before. But I want to post my solve in case anyone is running into this problem because it was driving me nuts.
Ran into the same problem, had 2D text, everything was scaled correctly and at highest resolution possible. I click it to become 3D layer and it would get soft.
So the problem was the position of the layer was not on a full pixel integer. It was on a number like 665.5 I changed it to 665, and it became crisp again as it should be.