Hi WendyMiller23,
Thank you for reaching out Microsoft Community.
You can only extend your Surface display using the Surface dock to 2 external monitors and connecting the 3rd monitor directly to the Surface device using an adapter will not work at the same time simply because the Surface device processor can only support 2 monitors at a time. You can only use the 3rd monitor if you are to daisy chain it to other monitor that is connected to the dock but daisy chaining is just simply letting you replicate the display of the monitor where it is connected to.
If this answers your question, feel free to mark this post as an answer. Otherwise, don't hesitate to respond to this thread if there's anything else that needs to be addressed.
Kind Regards,
Crest_Z