There’s a button on the back of your G3 in the INPUT section (far left when you look at the back of the G3) marked “Active” and “Passive.” MAKE SURE THIS BUTTON IS SET TO “PASSIVE” BEFORE YOU PLUG THE G3 INTO YOUR AMP OR PA. If the button is set to “Active” with a mic plugged into the G3, the resulting output signal will overload the amp or PA input; you’ll get heavy feedback at even low volume levels.

Zoom G3 back view, input jack and passive/active button at far left
Zoom G3 back view, input jack and passive/active button at far left

A simple thing, but important. So check the Input setting before you connect the G3 to the amp or PA. You’ll be glad you did.