VertexColorBlending

Hello there. I need help with this thing… I wanted my 3 materials(that is represented for now as colors) to be blend by 3 channel vertex color. But I can’t get it right( As u can see from second screen there is WHITE place for three color covered area, but I get only one channel result there wich blends last. Please share if it is posible, how to blend this three colors(materials in future) to get the same result as on the vertex paint preview, to be sure all place covered in right weight.