Multi-External Monitor issues

To support up to TWO displays on one cable on a Mac, the port, the cable, and the first device (Dock or Display) must be genuine ThunderBolt devices.

USB-C has only Half the data pathways provided by Thunderbolt, so the Mac does NOT consider it adequate for running two displays.

<<. I cannot get it to work such that it's just a single USB-C plug and play. >>

Your Mac is working as designed.

That is a USB-C Dock. On a Mac, that Dock supports ONE fully hardware-accelerated display with DisplayPort Alt Mode.

Additional displays are not fully hardware-accelerated. Additional displays require use of its built-in DisplayLink hardware, and the loading on your Mac of the DisplayLink Driver.

DisplayLink technology creates a "fake" display buffer in RAM, sends the data out over a slower interface to a stunt box with DisplayLink custom chips that put that data back onto a "legacy" interface. It is not a true "accelerated" display, and it can suffer from lagging. Just adding the DisplayLink Driver is not adequate to get a picture -- you need a DisplayLink "stunt-box" or a Dock that includes DisplayLink chips.

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It may be acceptable for a second display showing slow-to-change data such as computer program listings, stock quotes, or spreadsheets, but NOT for full motion Video, not for Video editing, and absolutely not for gaming. Mouse-tracking on that display can lag, and can make you feel queasy.

In a pinch, it may even play Internet videos (as one user put it) “without too many dropped frames".

If you are only doing program listing and stock quotes and other slow to change data, DisplayLink can work for you, but requires you to make some strong compromises.

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It is really nice to know that you can use a DisplayLink display if you MUST have an additional display for some of the types of data I mentioned. But that is NOT the same as the computer supporting a second, built-in, Hardware-accelerated display.

These displays depend on DisplayLink software, and are at the whim of Apple when they make MacOS changes. There have been cases where MacOS changes completely disabled DisplayLink software, and it took some time for them to recover.

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I think the Big Surprise for a lot of Hub/Dock buyers is that they thought they were getting a "real" display, but actually got a DisplayLink "fake" Display. If you got what you expected in every case, I would not use such pejorative terms to describe DisplayLink.

Compatibility—Compatible with USB Type-C and Thunderbolt 3 Macs (10.14+ and above, with driver installation), Windows ...

https://plugable.com/products/ud-3900pdz

Utilizing innovations introduced with USB 3.1, this dock takes advantage of "Alternate Modes" from the USB-C specifications for one of the display outputs and host charging.

This new and improved DisplayLink dock has been designed from the inside out to provide a triple-display dock that people can rely on.

...

Windows and Mac users can download DisplayLink drivers from Plugable that are verified to provide the best experience

Hello Azizen,

Let's use the steps below to make sure that your Mac is all up to date:

To check for new software, open Software Update. It's in System Settings (or System Preferences), which you can open from the Apple menu  in the corner of your screen:

• Choose Apple menu  > System Settings. Click General on the left side of the System Settings window, then click Software Update on the right.

• Or choose Apple menu  > System Preferences, then click Software Update.

If Software Update finds new software, click the button to install it. You will be asked to enter your administrator password. During installation, your Mac might show a progress bar or blank screen several times, and you should avoid putting it to sleep or closing its lid.

If Software Update says that your Mac is up to date, then no new software is currently available for your Mac model. Software Update shows only software that is compatible with your Mac. 

You can also find those steps in this resource: Update macOS on Mac - Apple Support Updating should help to further isolate a potential software conflict.

Should that persist, do you still experience those behaviors when your external displays are connected directly to your Mac without using a dock?

Let us know, and we'll keep moving forward based on your results.

from that same technical article:

HDMI 1 (Alt Mode) allows for a maximum output resolution of 4K 30Hz (3840x2160 @ 30Hz). Lower resolutions such as 1920x1200, and 1920x1080 (1080p) are supported at 60Hz. The HDMI 1.4 connection on this dock is provided by converting DisplayPort Alt Mode to HDMI 1.4.

HDMI 2 and 3 (DisplayLink) allows for a maximum output resolution of 2K 60Hz (1920x1200 @ 60Hz). The HDMI 1.3 connection on this dock is provided by the DisplayLink DL-3900 USB graphics chipset.

DisplayLink is a technology that allows compressed data of various types to be sent over USB data connections. The DisplayLink DL-3900 chipset in this docking station connects all the data and video connections, other than the "4K" HDMI 1.4 connection, on this dock.

There are a few important limitations of DisplayLink technology. DisplayLink uses a proprietary compressed video stream that is partially generated by both your computer's CPU and GPU, and is a "virtual" video adapter. Because the video is compressed and packetized as data, then reassembled by the DisplayLink chip, the pacing of new frames of video can be inconsistent, particularly if large portions of the content being sent through DisplayLink is changing (such as with video playback).

Because of how DisplayLink provides video, video outputs provided by DisplayLink technology should only be used to display content like office applications and web browser windows, not games or video content.

Azizen wrote:

Can you explain why there are tons of reviews on this docking system showing macbook's having up to 3 monitors then? Is it fake?

https://www.amazon.com/dp/B09NQQ1G1N?psc=1&ref=ppx_yo2ov_dt_b_product_details

Scroll down the page, and you'll find:

"For computers that only support one display, the UD-6950PDZ laptop dock has you covered with DisplayLink technology. Just a heads up, you’ll need to download a DisplayLink driver to get full functionality."

It's not giving you three first-class hardware display connections through a single USB-C link, regardless of the capabilities of your computer or the link.

Hi Azizen,

Based on the troubleshooting that has been completed up to this point, your best bet will be to Get Support directly from Apple from here on out. You can use the link here to get started.

Take care.