What tools do you use in academia to jointly review a thesis?

This is a good question and I don't want to discourage you, but let me explain what you will likely find most of the time.

They each already have a preferred way of giving you their feedback and some or all of them will not want to use whatever tool you choose. Since they are doing you a favor, you will need to respect their preference. For instance, I prefer to take a hard copy and a red pen and go to a coffee shop or library in order to focus.

Also, when I review a thesis I do not want to see comments from other reviewers before I form my own opinion.

Adobe Acrobat has a feature called Shared Review.

It lets multiple people in real-time comment on a single PDF so long as the PDF is hosted in a single place (e.g., if your department has network storage or there are services online to host such files).

It works pretty well and is already built-in.

Alternatively, you can email them each the PDF and then use the merge comments features to combine them, then send out that PDF. Not as nice but it doesn't have to be hosted.

Not really a tool for commenting, but very useful for review: if you go through many iterations, you might want to consider using a tool to generate a "track changes" PDF file, which shows the supervisors what parts of the thesis have changed since the previous version. Especially if you are at some point making many changes scattered throughout the thesis.

I would recommend latexdiffcite for this purpose. It's an improvement over the earlier latexdiff program. latexdiffcite even understands git and can give you diffs between different commits. Illustration: