M5 Pro MacBook Pro: Third display not detected

I ran into the same third-display limitation on an M5 Max and found a workaround that gives me three Studio Display XDRs (2026) at once, so I'm posting it here in case it helps anyone in this thread.

My setup: MacBook Pro 16" M5 Max (128GB), macOS Tahoe 26.6.2, 3x Studio Display XDR. Same symptom as described above — only two XDRs would come up, the third got no signal, and lowering the refresh rate to 60Hz in System Settings made no difference. This matches what you observed: macOS seems to allocate display resources at link training time based on the display's EDID-declared maximum (5K@120Hz), not the mode you actually select.

What worked:

1. Plug a 4K UHD 60Hz HDMI dummy plug into the HDMI port FIRST, before any display is connected. (Critical.)
2. Connect the three XDRs, leaving a short pause between each. Both topologies worked for me: all three directly to the Thunderbolt ports, or all three daisy-chained with a single cable to the MacBook.
3. At this point only one or two of the three will be on. NOW remove the dummy plug. (Just as critical as step 1 — all three will not come up while the dummy is still attached.)
4. The moment the dummy is removed, the remaining display(s) come up.

Result: one XDR at 120Hz, the other two at 60Hz. This survives reboot and sleep/wake, and works both in clamshell mode and with the lid open. My daily setup is now all three daisy-chained with one cable.

My understanding of why it works: with the dummy taking up part of the display budget, the XDRs negotiate at 60Hz during link training instead of demanding a 120Hz-class pipe. Removing the dummy then frees that budget so the remaining display can come up, while the already-negotiated links stay as they are.

For anyone else trying this, these did NOT work for me: setting 60Hz in System Settings and reconnecting, Thunderbolt 5 docks (they don't raise the maximum display count), and changing connection order without the dummy plug.