Your M3 MacBook Pro will be able to support multiple displays soon, just like the M3 MacBook Air

MacBook Air M3.

Users of the M3 MacBook Pro are going to get one of the biggest new features of the M3 MacBook Air with a software update — native support for multiple displays.

The same limitations apply, however, Apple confirmed to 9to5Mac. That means you can only have the two displays active while the MacBook is closed — so your second screen is either your external monitor or your MacBook display.