USB-Mikrofone funktionieren nicht

Actually, I don't think it's likely to be an Audition problem at all. This has almost certainly to do with how your Mac arranges to synchronise the USB sources. What has to happen is that one mic is selected by the aggregation system to be the 'master' and the output from the other ones are synchronised to that - so that you can record multitple sources without large clicks occuring when the synchronisation drifts. This is fundamentally what an aggregation system's purpose is.

Now, as far as I'm aware this will work for multiple devices, but there is apparently one limitation when it comes to microphones - and that is that it will work fine if all the mics are different, but not necessarily if two of the three are the same type of mic. The word is that you need three different USB mics for this to work.

Now, this is what others have said. I cannot confirm whether it's true, but it does make sense because of the way that USB devices are identified. The other thing that might make a difference is how you are connecting the three mics - I can also see potential difficulties if they are connected via a hub, as this has been reported to be the cause of several USB issues in the past.