How do I get the average color among 3 points in a texture?

If its offline (simplest and shortest code):

• Make a loop that covers the bounding box of the triangle

• Check each point if its inside the triangle, if yes accumulate and keep that count.

• When the loop end divide the accumulated RGB value by the number of points. Done.

Pseudo code:

 x0 = min(p0.x, p1.x, p2.x); x1 = max(p0.x, p1.x, p2.x);
 y0 = min(p0.y, p1.y, p2.y); y1 = max(p0.y, p1.y, p2.y);
 color.rgb = 0; count = 0;
 for(y=y0; y<y1; y++)
  for(x=x0; x<x1; x++)
   if(IsInside(float2(x, y), p0, p1, p2)) { color.rgb += texture.GetPixel(x,y); count += 1; }
 
 if(count>1) color.rgb /= count;

See this if you need the barycentric logic for IsInside(): http://www.blackpawn.com/texts/pointinpoly/

If this is done at runtime and you need optimal performance will need to write a simple triangle rasterizer and use that to accumulate value.

note: one important thing. If the texture hold gamma compressed values you 'cant' accumulate the value without first decompressing them. They should be actually sRGB encoded, so use the opengl sRGB function to decompress to linear before accumulating. Then after you do the average, you need to compress back to sRGB. Follow the formula in this link to decode the texture RGB color data. https://www.opengl.org/registry/specs/EXT/texture_sRGB.txt

Using Texture2D you can use GetPixel to get pixel color from specified position. Then You just add all color values (r,g,b) and divide them by their count. Example, you have 3 pixels:

 Color c1;
 Color c2;
 Color c3;
 Texture2D tex;
 
 void Awake()
 {
  c1 = tex.GetPixel(0,0);
  c2 = tex.GetPixel(0,1);
  c3 = tex.GetPixel(1,1);
 
  float Raverage = c1.r + c2.r + c3.r;
  float Gaverage = c1.g + c2.g + c3.g;
  float Baverage = c1.b + c2.b + c3.b;
  Color average = new Color(Raverage / 3, Gaverage / 3, Baverage / 3);
 }

Hope that helps,
Paul