In 2.0 there's a choice called Options>Monitoring>Audition Mix>Smart Input. I assume this is carried on in 3.0, which I haven't looked at yet. Anyway, "Monitors the input only when recording the track. When playing back, monitors only the clips on the track, not the input."
If, instead, you go to Options>Monitoring>Audition Mix>Always Input, you don't get playback.
I'm pretty sure you need to enable the ASIO drivers for this to work. I have reverted back to 1.5 (to preserve compatibility with another person who has CEP -- the .ses formats are not the same, beginning with 2.0 and, while 2.0 will convert earlier versions, the earlier versions can't read them in the 2.0 format), which does not use ASIO. I use a hardware digital mixer with its own monitoring routing, so it's not an issue with my setup.
I hope this helps...I would have responded sooner but I wasn't sure if 3.0 was the same.