Best Route For Me | Multi-Site or Multi-Theme?

Having a difficult time deciding the best route here and would like input, please. Anything you can give is sincerely appreciated.

My Situation:

We have 3 different themes (1 is for the site, 1 is for the support ticket system, and 1 is for a user submission segment).

The other option I need is for users to only login once to access it all.

Recap:

Users login to one site that has 3 different themes (each with their own widgets).

Best Solution I’ve Come Up With:

The Multi-Theme Plugin here:
https://www.ads-software.com/plugins/jonradio-multiple-themes/

I’m wondering if a multi-site would just be easier or if there is another path that I’m completely missing?

Thank you in advance for any knowledge, advice, and help.