How to use 3 monitors on MacBook Pro that does not have a graphics card
Officially Apple only supports two external displays on this model.

More specifically, it supports an HDMI-compatible device, including 4K, while using one Thunderbolt display or it can support up to two Thunderbolt displays.

In terms of external resolution this model supports up to 3840 x 2160 pixels at 30Hz or 4096 x 2160 pixels at 24Hz via HDMI and up to 3840 x 2160 pixels at 60Hz via Thunderbolt.

You could investigate external graphics card options, but I would suggest living with the two displays instead. However here are some resources that may help you decide:


https://bizon-tech.com/us/bizonbox2s-egpu.html/
http://www.xlr8yourmac.com/tips/MBP_ThunderBoltVideoCard.htm
http://www.techrepublic.com/article/how-to-build-an-external-gpu-for-4k-video-editing-vr-and-gaming/