in my experience the official dock has trouble with just one monitor. i couldn't imagine trying to configure 3.

when i boot into desktop mode with the official dock, there's a 50-50 chance that it just doesn't load the image at all on the monitor and i have to take it off the dock and just use desktop in handheld mode.

maybe a better splitter or connection could do it. but the official dock seems to have a lot of issues with USB inputs in general.

I think it would struggle to have the bandwidth to pull that off, but maybe if you keep the resolution and refresh rate down. And that's assuming USB-C to USB-A would work in this case, which I don't know the answer to.

I assume the tablet gets its video through standard HDMI/DP over USB-C, hence a USB-A to C adapter wouldn't work.

If you still want to use the tablet with the official dock, there may be HDMI+USB -> USB-C adapters capable of inserting the HDMI signal into the USB-C connection. I'm not too knowledgeable on this topic, so you may want to resort to some more research of your own.

~~Also keep in mind (as far as I'm aware) there's a hard limit on 2 external monitors, unless you use some "trickery" like DisplayLink.~~

Edit: I've done a bit more research and I'm pretty sure that more than 2 monitors are supported. There are two limiting factors here:

1. The GPU in the Steamdeck (custom APU which I didn't directly find a spec sheet for), will typically have a max of 2-4 displays
2. The physical connection, AFAIK as the Steamdeck only has one USB-C port, this means it has one DisplayPort 1.4 link available. This link can be divided into multiple video streams. I couldn't find a table for DisplayPort 1.4, but this website has one for DisplayPort 1.2, suggesting you should be able to run four 1080p60 signals at the very minimum.

Now with all this information, as long as your Steamdeck can output to any three screens (ie. two external ones and the internal steamdeck screen), it should also be possible to output three 1080p60 signals to external monitors.

So the setup is currently two external monitors? Or does that include the Deck monitor? Is the USB connection to the Wacom just for pen input or does it transfer image as well? If USB-C is used as the monitor port it most definitely will not work with USB-A of any kind. Not even USB-A 3.1. You either need a different dock with a USB-C port or you need the Wacom Link Plus (which means you probably also need a different dock with at least 2 HDMI ports or one HDMI and one DisplayPort).

You might be able to get this to work, but not in the way you are currently thinking (and it would be kind of janky). The Tablet needs a USB-C port with Displayport alt mode, so a USB-C port that can also transmit Displayport signals. Not every USB-C port and no USB-A port can do that. But Displayport supports daisy chaining multiple monitors from one port on the device. With multiple (pretty expensive) adapters you might be able to do it. You would need a Displayport daisy chain adapter (a few Monitors have this built in) to get two usable ports from the single one on the dock and one that converts a Displayport and a USB-A port to USB-C with Displayport alt mode. But this might still not work because of compatibility problems, so I would only try this, if you have free returns.

An external GPU may be the way to go. Doesn't have to be a beast of a GPU. Probably best to find one thats AMD for easy compatibility. You could also find one that has the amount of connections you need.