I have to replace a 3 way dimmer in my dinning room. Please tell me how can I identify the common wire?
Thank you

The common on the 3 way switch will be a black or very dark screw, while the other two screws for travelers will be brass.Many switches will even have the screw marked "common".

The dimmer common will usually be a black wire, and the travelers will be two red wires. The instruction will explain how to connect the dimmer.

The instructions will also demand the power be turned off while installing the dimmer, do it because they mean it, connecting these devices while the power is on will damage the unit, and you.