my few thoughts:
1. The E30 is really a tough decision since it's been with you for so long. I take it the sentimental value is more than the $ value of the car. The car probably won't appreciate for at least another 20 years, but I think it'll retain it's current value for at least the next 5 years. Hard for me to tell you to sell it.
2. E90 - keep it man, keep it.
3. X3 - i think you should sell it and drive the E90 in the winter
4. JWC Mini. I drove one extensively in an autocross event last summer. It was very fun. But I have to say, if you want to fall back in love witht he M3, take it to the track. Or do an autocross, it'll confirm with you why you bought the car. I would never in my life consider selling the E90 M3 for a JWC. Even if i had the funds to simply add the JWC to my garage, I'm not quite sure how much I would drive it, the M3 to me is the best "all-rounder".