Dell precision 5560 can't do 3 monitors. RTX card not primary graphic controller

Solution 1:

All the information you found simply does not apply to your device. Instead, check the correct document: precision-15-5560-laptop_owners-manual_en-us.pdf. On page 20, you can find the “Multiple display support matrix”. It’s supposed to relay how multiple displays can work.

My interpretation: To get the number of displays you want, you must use a Thunderbolt docking station. Most docking stations are not using a Thunderbolt connection but only USB-C with DisplayPort Alt Mode.

And indeed, when using my TB16 (Thunderbolt, connected to 2 2560×1440 displays) docking station and the WD15 (DisplayPort Alt Mode, connected to 2 1920×1080 displays) docking station from the neighboring table, I can drive four displays in total with my Precision 5560. That means one of the smaller displays remains off so my laptop display can remain on.