If you disconnect your Mayfly board from the computer, does COM3 still show up as an active device on your computer? I’m guessing that COM3 is actually another com port on your machine, like a bluetooth or other USB device on the motherboard. If your Windows computer correctly identifies the Mayfly board, it will shows up as “Silicon Labs CP210x USB to UART Bridge”. If you’re not seeing it being added to the list of USB devices in Device Manager when you plug it in and then disappear when you unplug it, then Windows isn’t seeing the Mayfly as a valid USB device. This is usually cause by a bad connection or wire in the USB jack or cable. So try using another USB-C cable (preferably a nice heavy-duty one and not just a generic charger cable. A Mayfly board can still be powered on and function normally with just the power wires in a USB cable. It needs 2 data lines in the cable to actually achieve the communication part between the Mayfly and the computer, and those wires can be easily broken.
If your computer still doesn’t see if after swapping cables, then check to see if it’s being seen as an unknown device (or Other Device). Windows should detect the Mayfly v1.x as a CP210x device and automatically install the correct drivers, if you’ve got your Windows Update setting configured to either automatically install drivers for new devices, or to prompt you for permission to install the drivers. But sometimes Windows is locked down so that no drivers are installed and no prompts pop up, so you have to manually download the driver from the SiliconLabs website, or point Windows driver update tool to the correct location. I can give you more instructions on how to do that, but the majority of the time swapping to a new cable fixes the issue if it’s not driver-related.