When driving I normally have my phone connected to the stereo and my radar detector and all seems fine. When I add a 3rd (watch or another device) i start getting issues. Can i connect more than 2?
Welcome to Android Central! It may have to do with what they're connected for. If you have the watch set to make and answer phone calls, then that might compete with the car stereo.
Bluetooth has a few modes - media, phone, file transfer. You can't have 2 media devices connected at the same time, you can't have 2 phone devices (a headset set as Phone is a phone device) at the same time, etc.