Hi @radconnorcon ,
According to this link the microphone is on the charge port flex cable.
Here is a link to the ifixit Oneplus 3 Teardown guide. At 4:11+ minutes into the video the microphone is accessible.
You could always try gently (stress gently) permanently removing the microphone from the cable and then re-assembling the phone. (I doubt that the microphone would be able to be reinstalled successfully - you'd most probably have to install a new charge port + microphone flex cable)
Hopefully the phone should still work if you use the earphones with the microphone attachment if you feel the need to make or receive calls.
Alternatively you could just simply block the phone's voice inlet hole on the bottom of the phone with glue or perhaps as a temporary test measure, clear adhesive (sticky?) tape to prevent sound reaching the microphone. This should achieve the same result with less potential damage from having to open the phone etc, occurring
Glad I'm too old and hopefully considered not interesting enough for it to be worthwhile to be bugged by anyone. Life's simpler this way. Cheers ;-).