I already had 2 external Monitors running from my MacBook by using 2 x miniDP to HDMI adapters.  I was using the fairly standard ones that come with the Mac but I have another Monitor which i used for my Lab PC (Windows) and Figured I could connect this to my Mac and run Microsoft Remote Desktop to connect to it, removing the extra Keyboard and mouse off my desk.

I have read a few articles saying people can and cant get the their third monitor working. Now the MacBook Pro has 2 x miniDP connections and a HDMI Port on the side, so i tried to connect my Third Monitor to this, no luck.  Next thing i tried was to order a USB to HDMI Converter, this is supposed to use a chunk of CPU cycles but i figured it was worth a try – No Luck

So I read somewhere that the MacBook mid 2015 has an additional graphics card which should help with this functionality, so i checked and it does; an AMD Radeon R9 M370x

My Mac Specs:

MacBook Pro Retina Mid 2015

So i found another post that mentions you need to have an ACTIVE minDP cable.  So for my 3 Monitor setup, I should be able to get away with having just 1 Active Adapter.  I order this to give it a try:

Plugable Active Mini DisplayPort to HDMI Adapter

Works a treat!

So in summary, to run 3 External Monitors with a Mid 2015 MacBook Pro Retina, you need to have at least 1 x Active miniDP Adapter.  I am running this via 1 x standard miniDP to HDMI Adapter, 1 x Active miniDP to HDMI Adapter and 1 x HDMI straight to the Mac.