From a technical POV I guess they share the antenna & at least the Pi3 is only capable of 2.4GHz Wifi so that's probably the reason for interferences. Maybe the Pi3+ & 5GHz Wifi will work better.
i got the 3+ in the retroRoom i'll give it a show in a few and either edit/Reply back w/ results. my non + is the one setup w/ a Waveshare 7' screen
edit @ 3:36PM : yea the 3+ can w/ Wifi on but just ever so bearly. i can hear if im listening for t a slight skip but thats it. i dont think the RaspberryPi's Bluetooth was ment for Audio in general IMHO.