All the answers given here are good, but as said by others, you need to ascertain why/what is causing the excessive wear first.
From what I've read and gathered, this is a very common problem on the Z3 and can be from minor to horrendous.
I had a similar problem, in that my wear was mildly aggressive on the inside edge only of the rear left, and incredibly aggressive on a very small part of the inside edge of the rear right.
I have no broken springs and the car appears to be stock standard in height.
What I have read and gleaned from a lot of other more knowledgeable posts, is that as much as camber comes into this, it's in particular (in my case) a toe out problem.
Blacklane and others suggest the weld in plates and yes, they are a totally one fix solution, so you really can't go wrong. It's just that some like myself aren't quite ready to have these attached to the car... yet.
I really do suggest that in looking to why you have this problem, you also consider a couple of things.
1. the bushes, not only in the rear trailing arms, but in my case the rear beam bushes (the 2 big ones) were excessively worn, allowing the rear to squirm around and (I believe) cause unusual alignment and wear on my inside wheels. - I'm very happy to be corrected here by knowledge people.
2. as explained to me by at least 2 BMW savvy mechanics, that the root of the problem (if the bushes are good) is that something could be bent and the culprit might be the rear trailing arm itself. Only problem is, I've yet to come across a reasonably priced new trailing arm!
So, in my case, I fixed the rear beam bushes first and not only did it change the feel and driving of the car instantly, my excessive wear on the rear right, appears to have slowed down (very subjective of course, but I started with new tyres originally and in a very short time one of them had chewed out, but not now).
I still intend to change the rear trailing arm bushes, but in my case I decided to see if the Powerflex adjustable eccentric bushes would give me just enough control to at least slow down the excessive wear to a manageable level and then maybe I just change the tyres off the rims if I really want to extend.
That's my course of action at the moment and if that all goes pear shaped, then either new rear trailing arms, or the weld in plates, but at the moment, just fixing the rear beam bushes in the rear seems to have helped and that's a good start, but good luck with your odyssey and I'd certainly like to hear how you finally nail the problem.