Dear Julie,
adding a third-level submenu would require some serious changes in the theme’s template file, which I usually classify as theme customization and thus have to ask a fee for. In case you are fine with this arrangement, please contact me via email and I will provide a quote.
To give my humble opinion, in cases where the top-level and second-level menus contain small numbers of items it is not recommended to create third-level menus as every additional navigational layer has increasing tendency to confuse visitors. On your website, this usability “rule of thumb” may also be topical.
Sincerely,
Andrew | Satori Studio