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No camera detected in Lightroom 3 with a canon D20.
Only Lightroom Classic has the capability for tethered shooting. I assume you are not referring to the old (as in ancient) Lightroom 3, are you?
If you are referring to that version of Lightroom, and you are referring to the Canon 20D, then you should be able to tether that model.
Here is a link to the troubleshooting page. If you are using the old Lightroom 3 then you will have to disregard the comment to be sure you are using the latest version of Lightroom classic.