Do you see COM3 in printer settings if you open port selector? That means the port is there and working (assuming it belongs to your printer).
Leave protocol on autodetect and then try different baud rates until it succeeds.
If you could select com and get a error message you have most probably a different software using the same port. Could be repetier-server if you configured it to use the printer. In that case do not select a serial port as connector type but repetier-server in repetier-host. Then you print through the server which acts as printer spooler for you allowing to close the host without stopping the print or controlling it with you smartphone using the server url.
Repetier-Software - the home of Repetier-Host (Windows, Linux and Mac OS X) and Repetier-Firmware.
Repetier-Server - the solution to control your printer from everywhere.
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