The 3806 allows you to assign the Surround Back amps to bi-amp the mains or to power speakers in Zone 2/ Zone 3.
First, check if the amp is assigned correctly. This can be done in the Option Setup screen's menu choice "1. Power Amp Assign". Make sure it says "S. Back" here. Once you confirm the above, you are ready to re-run the auto calibrate function. Here too you can change the power amp assignment. And here too you need to make sure it says "Surround Back". With these two settings in order, start the auto calibrate routine.
This time it should detect the surrounds back speakers.