Start with evidence
Run the tests that separate the likely branches
Screen Resolution TestShow browser viewport, screen dimensions, and device pixel ratio.
Run →Monitor Refresh Rate TestEstimate display refresh timing using animation frames.
Run →Display Raw Bandwidth CalculatorEstimate raw uncompressed pixel data rate from resolution and refresh rate.
Run →USB-C Display Path Readiness CheckCheck four documented prerequisites for sending video through USB-C, Thunderbolt, a dock, cable or adapter to a display.
Run →Decision table
What the observation changes
ObservationWhat it meansNext action
All three displays work individuallyThe displays themselves are likely functional.Rebuild the multi-display topology and verify dock/output limits.
Third display works direct but not through dockDock/adapter path is the stronger suspect.Check the dock's actual multi-display/Thunderbolt capability.
One display fails even aloneThis is not primarily a display-count problem.Test that cable/input/display independently.
Sequence
Work from reversible checks to escalation
- 01
Confirm M5 Pro chip in About This Mac.
- 02
Test each monitor direct and alone.
- 03
Check resolution/refresh combinations against Apple support.
- 04
Add dock/adapter outputs one at a time.
Likely areas
Keep these branches separate
Dock topology/bandwidthUnsupported resolution-refresh combinationCable/adapter limitationWrong chip/configuration
Repair boundary
When to stop changing settings
If supported direct combinations fail after known-good cables/displays, use Apple support or the dock vendor as appropriate.