Hi there,
Thanks so much for using Tortuga.
You have to create a child theme and basically do these three things:
1) Register a new sidebar in the backend in the functions.php of your child theme
2) Overriding all template files to show the sidebar on the frontend
3) Override the whole responsive styling of the theme to show a three column layout instead of a two column layout
I think you need about 4-5 hours of coding work as an experienced WordPress theme developer for this task, so this is not a minor modifications.
I guess it is easier to just choose a theme with two sidebars and a three column layout in the first place, rather than completely changing the layout of an existing theme. You can take a look at our Gambit theme: https://www.ads-software.com/themes/gambit/
Cheers,
Thomas