I'd repot now before too much growth begins in earnest. And you may find that it will naturaly want to divide itself with the root mass breaking into chunks. Personally, I'd find that to be a good thing as you can then have at least two containers of the same plant (if not suited for direct inground planting)! And you will want a smaller root mass overall if you wish to replant in the same container. I'd suspect root congestion after 3 years was just as much an issue for sparse flowering as the need for fresh soil.

After reading your post gg I went out and repotted two containers of overwintered mums. Thanks for the reply.