Did just now, with much the same results. Agreed, it's a very scanty test but it behaves better with these flags than not.
Wikipedia says about OpenGL 3 versions:
3.0 August 11, 2008 GLSL 1.3, Texture Arrays, Conditional rendering, Frame Buffer Object (FBO)
[26]
3.1 March 24, 2009 GLSL 1.4, Instancing, Texture Buffer Object, Uniform Buffer Object, Primitive restart
[27]
3.2 August 3, 2009 GLSL 1.5, Geometry Shader, Multi-sampled textures
[28]
3.3 March 11, 2010 GLSL 3.30, Backports as much function as possible from the OpenGL 4.0 specification
It seems 3.3 version is some kind of precursor of OpenGL 4.
Back in July-2020, Parallels team posted this about Parallels 15 saying it only supports OpenGL 3.2:
https://kb.parallels.com/115487
So, I believe by setting video.gl3=2 in Parallels 16 you are forcing it to run with OpenGL 3.2 which was the default in Parallels 15.
In theory, anything that runs ok in 15 would be running ok in 16 with this boot flag (hopefully).
This is at lease my conclusion after exploring this.