Yes, it is great, isn't it? Are you asking how to make it work? You have to set a setting:
For ReStructured Text, the setting is @bool vr3-math-output = True
For Markdown, the setting is @bool vr3-md-math-output = True
You also have to tell it where the MathJax script file is located. The setting is
It is better - faster to render, and you don't need an internet connection - if you download the files to your own computer. Then the url would be something like this:
file:///D:/utility/mathjax/es5/tex-chtml.js
This information and more is in the VR3 Help, which you can display from the Plugins/Viewrendered3/About menu item.
Once you have the settings set up, you don't need to write the "$$", etc. if you are usng RsT. For math in RsT use standard RsT syntax for math:
.. math::
\sum_{n=1}^\infty\frac{-e^{i\pi}}{2^n} + (\frac{5 - \frac{1}{x}}{4})
α_t(i) = P(O_1, O_2, … O_t, q_t) = S_i λ
For Markdown, you do use the syntax you showed:
$$\frac{2 \pm \sqrt{7}}{3}$$