Connecting dual monitors on Mac Air M3

If your docking station is a "plain" USB-C one, that could be an issue. Macs do not support MST in the same way that Windows PCs do, and aren't willing to drive multiple monitors over a single USB-C (DisplayPort Alt Mode) link. Often people who connect two monitors to a plain USB-C dock report the same "can't do anything but mirroring" symptom that you do. (I.e., the second monitor isn't blank, but it's not getting an independent signal, either.)

To connect two displays to the same Mac host port,

• The host port must be a Thunderbolt one
• The device plugged into must be a Thunderbolt device (such as a Thunderbolt dock or hub)

Note: The M3 MacBook Air can only support two external displays when its lid is closed. I'm not certain how that might complicate matters if you wanted to keep your displays plugged into a dock at all times.

This may help:

Use dual monitors with your MacBook Air and MacBook Pro with M3 chip - Apple Support

Regards.