I'm currently using 3 of the cheap dongles to get audio from tapped radios into DSD+ etc ...
I've re-read that one sentence like 10 times now and I still don't get it, must be getting old or something. If you're using 3 RTL-based sticks does that mean you also have tapped radios too providing completely different audio signals to DSD+ or is there just 3 total tuners period?
After an 11th read I'm wondering if you mean you're using USB
audio dongles aka USB "sound cards" and not RTL-sticks... if that's the case then yes I now get it.
And after thinking about it some more that's going to be some rather complicated device - a "7.1 USB dongle" means it's designed to output 7.1 digital audio aka Dolby Digital or DTS potentially to a sound system receiver, not that it has 7 inputs but who knows what modern technology has come up with these days.