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.