Reference:Triangle Texture Interpolation

This feature is utilized in a number of visualization approaches: triangles with individual textures for each vertex, which are interpolated during rendering.

Syntax:

MESH_TRIANGLE:
triangle { 
  <Corner_1>,
  <Corner_2>,
  <Corner_3>
  [MESH_TEXTURE]
  }   |
smooth_triangle { 
  <Corner_1>, <Normal_1>, 
  <Corner_2>, <Normal_2>, 
  <Corner_3>, <Normal_3> 
  [MESH_TEXTURE] 
  }

MESH_TEXTURE:
  texture { TEXTURE_IDENTIFIER } |
  texture_list {
    TEXTURE_IDENTIFIER TEXTURE_IDENTIFIER TEXTURE_IDENTIFIER
    }

To specify three vertex textures for the triangle, simply use texture_list instead of texture.