Fragment shaders; How do you get data from all three vertices of a primitive?

I’m very new to writing shaders, after writing my own simple shader using vertex colors and flat shading I ran into a problem that I couldn’t find an answer to;
Is it possible to get data, specifically the vertex color, from all three of the vertices that make up a primitive and then modify it before sending it to the fragment shader?

Yes, it is possible with a Geometry Shader Geometry-Shader Object - Win32 apps | Microsoft Learn . An example for Unity can be found here: