avrdude: ser_open(): can't open device "\\.\COM3"?
I have experienced a similar issue after plugging in another Arduino device. I didn't set up my environment for a long time and thus forgot he iterates the COM-Index. Maybe this is too easy, but you didn't provide much information.

This solved my problem:
Open Arduino application > Tools > Port > Change to the target Port

If you are unsure which port to choose from: (on Windows)
Open Device Manager > Connections (COM &amp; LPT) > Usb Serial Port (COM[x])
where "[x]" is the index you are looking for.