Hi Nedlaw, i use an xr18 on occasion and have tried to see if i can help, my understanding of ST3 is if you choose main on any track it sends it to input 1/2 of XR18 and if you choose AUX on ST3 it sends the audio out to input 3/4 on XR18, now assuming i got that correct, this is how you setup the XR18 to place the AUX of ST3 to chanel 17/18 of XR18.
On xr18 select chanel 17/18, go to input , select USB input and choose USB source as usb 3/4, set up your gain trims as required.
I am hoping this will work for you , i havent had a chance to try it yet myself.
Cheers Damir
After reading the online manual i can see why you need help as it tells you WHAT the selections do but it does not tell you HOW the MAIN AND AUX selection actually outputs to external devices so you can then setup your devices.
Hopefully Peter will correct that in the manual so others can work it out.
The reason i found out is Peter informed me of how the audio output was actually spat out, i just hope i remembered it correctly.