MacBook Air M3 external display resolutions

The M3 MacBook Airs support

• One external display with up to 6K resolution at 60Hz
• Close the MacBook Air lid to use a second external display with up to 5K resolution at 60Hz

That's directly from the Technical specifications.

When you are running a screen in a Retina scaling mode, the internal canvas has 2x the number of pixels (in each direction) as the "UI Looks like" resolution selected in Displays Settings.

My guess is that the 6K and 5K limits apply to the resolution of the internal drawing canvas. If that is the case, it would suggest that you can run your second monitor

• At the Retina "like 1920x1080" and "like 2560x1440" settings – where the internal canvas resolutions would be 3840x2160 and 5120x2880, respectively.
• At the non-Retina 3840x2160 setting – where the internal canvas resolution would be 3840x2160.

but not

• At the Retina 'like 3008x1692" or "like 3360x1890" settings, where the internal canvas resolution would exceed the 5K limit on the resolution of the "close the lid" display.

I could be mistaken, but I believe that all of the MacBook Pros with M1 Max, M2 Max, and M3 Max chips can drive three external 6K displays at the same time they have the lid open. So you needed to use up two "6K" outputs to run two 4K monitors in "like 3008x1692" mode, a {M1/M2/M3} Max would have had the hardware to spare.

If you run System Information (Option- / System Information…), and look under Graphics/Displays, it will show you "Resolution:" and "UI Looks like:" figures for your current display setting. Here you can see that it thinks my 3840x2160 monitor, running in Retina "looks like 2560x1440" mode, has 5120x2880 resolution. (Drawing takes place onto a 5120x2880 canvas and then the system downscales that to fit the actual 3840x2160 screen.)

If you change the settings while System Information is open, you can use File / Refresh Information to update the display.