Hi, Ted. Just reporting that I have used it in X3 and it worked as expected... *mostly. I installed X3 over top of X2 (and X1) and it showed up in my VSTs without any fiddling (just saying that because maybe you installed it in another manner).
*When I say "mostly" one thing that did happen was that after a while of screwing around with it it would simply stop responding to any changes I made to it. Like I would drag or resize the box/circle or screw with the faders and it simply wouldn't do anything. When this happened I'd have to delete the instance of R-Mix and reload it... then I can mess with it for a while again.
No graphical issues but yeah... something weird is going on. Fortunately it is a simple enough program that it's not hard to manually reproduce settings but I'll have to remember to bounce to a new clip to save my work instead of relying on it to remember on it's own.
Probably not helpful but thought I'd mention it.