def three_tone
        self.new({:type => :min, :val => 0, :color => 'FFF8696B'},
                 {:type => :percent, :val => '50', :color => 'FFFFEB84'},
                 {:type => :max, :val => 0, :color => 'FF63BE7B'})
      end
A builder for three tone color gradient
@example
#this creates a three tone color scale
color_scale = Axlsx::ColorScale.three_tone
@see examples/example.rb conditional formatting examples.