Macbook Pro 15" cannot use both thunderbolt at same time

I'm trying to connect three external monitors to my Macbook Pro 15".

I connect one monitor to the HDMI, and the other two to Thunderbolt.

Only two monitors will give me a picture.

If i disconnect one working monitor, the one who was not working will begin to work.

I can change ports between the monitors and always only two monitors will be working.

I can open my Macbook and have the inbuild screen to work as a third screen.

But the external screens will only show two at time.

Why is that?

I'm using El Capitan.

I have tried the "Search display" function in settings.

I have also tried to reboot.

I have also tried to unplug the power cables for the screens.