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.