It's unusual to have an Uno with the CP210x; I don't say it's impossible. If you disconnect your board, does COM3 disappear?
Is anything connected to pins 0 and 1? Disconnect that and try again.
Is anything connected to other pins? Disconnect and try again.
Can you give a history of the board? Is it a new board? Or did you use it before? Did it work in the past? What is your current project about?
You're using IDE 2.x but posted in the forum section for IDE 1.x. I've moved your topic to the section that is more suitable for upload problems.