Mac book pro not showing third screen

[mark_matrix]

Hi, I wonder if someone can help, I have a 2020 Macbook pro running Montery.

I have the installed this software and I have two external Samsung monitors linked using a Dell display link box.

Currently it only recognises it as one screen, so i have my mac screen and the two others but it just extends display to one screen then mirrors onto the second one. any ideas on how to set this so that it has three separate screens?

Ive looked in the mac settings and it thinks its just 1 external screen not two.

Thanks
Mark