Working with 2 monitors and a 4k TV

Hej there. I have two LG ultra HD 4K 27 inch displays connected to my Mac Pro (late 2013). When I work with video editing software I need a another display, so I bought a Samsung Ultra HD 4k TV. The problem is that it never works with the 3 displays together. When I connect the TV (via HDMI) one of the LG displays disappear and I just can´t find a solution. The 2 LG´s are connected to the Thunderbolt inputs in the computer. When the TV is disconnected the 2 LG displays works fine again.....

I really look forward to an answer.

Thanks

Christer Skoglund

Canarian Islands