How to enable third monitor in Macbook Pro 15'' mid 2015 model?
Apple doesn't support this configuration.    Per the Apple specifications, you can support up to two external monitors:


  Dual display and video mirroring: Simultaneously supports full native
  resolution on the built-in display and up to 3840 by 2160 pixels on up
  to two external displays, both at millions of colors


(Emphasis added)