Can you charge a Quick Charge 3.0 phone with a Quick Charge 2.0 mains adapter?
From  Qualcomm product manager, responsible for Quick Charge


  Of course, QC3 is backwards compatible to QC2 so your existing QC2 adapter will charge a QC3 phone and vice versa


(Emphasis supplied)

Thanks for asking this, I was under the impression that it is one way compatibility