The Dell P2422H is a 24" 1920x1080 pixel monitor with a refresh rate of 60 Hz. It has DisplayPort, HDMI, and VGA inputs. It also has a USB hub (one upstream port, four downstream ones).
Use dual monitors with your MacBook Air and MacBook Pro with M3 chip - Apple Support
Your Mac can support one external monitor when the lid is open, two when the lid is closed.
Macs will only drive two monitors over one host port if that port is a Thunderbolt port, the device attached to it is a Thunderbolt device (such as a Thunderbolt dock, hub, or dual-display adapter), and the Mac supports driving two (or more) monitors, total, over all of its USB-C or Thunderbolt ports. Since you are using two USB-C to DisplayPort adapter cables, it sounds like you are attaching your monitors to different host ports.
There are only a few things I can think of:
- Try plugging in the cable for the second monitor after closing the lid of the M3 MacBook Air. (This is how Apple says to do things in the Support document.).
- Check the input selection on any monitor that is "blank" to make sure that it is showing the video signal from its connected input (instead of the "signal" from one of its unused inputs).
- Test both monitors and both cables individually to rule out the possibility of one being bad. It sounds as though you have tested both cables, but it wasn't clear whether you have tested both monitors.
If all of the above fails, and you still can't get things to work when the M3 MacBook Air's lid is closed, and your monitors are connected to separate ports, your monitors may be incorrectly sharing a serial number. This happens when manufacturers are tempted to take shortcuts during manufacturing, and load entire batches of monitors with the same serial number, so they don't have to individually program serial numbers for each one.
It would be nice if you could connect the monitors one at a time, get screen shots of their serial numbers – and then compare the numbers to confirm the presence or absence of this issue. Unfortunately, I'm not sure how you would get the Mac to display those numbers.
