Several WPMU domains

I've had an issue that I solved my own way. I have a wpmu website where I want users to be able to choose from 3 domains. However, I don't want a signup to block up the blogname on all 3 domains at once.

So when someone signs up for I don't want them to block up for someone else to register. I tried different things for this but in the end I used 3 separate installs with customized signup pages. The downside of this is that you would have to re-register to have 2 blogs on separate domains.

Anyone had this concern?