Stock rims were probably 1.60 front and 1.85 rear. I run 1.85/2.15 on my TZ350 and 2.15/2.50 on the RZ350.
If your tires are OK on those fat rims, keep them. I think it should look cool. Let me think for a moment. A 3.00 tire is really a 75/75-16 or nearest equivalent is probably 80/90 if they made such a thing. 80/90 AVON 18" fits on a 1.85 rim or 1.60 to 2.15 range is acceptable. 90/90works on anything from 1.85 to 2.50, so the tires will probably be OK.
What tires did you choose, and what rim size do they recommend?