iMac connecting to two monitors Problem

Hello.

I have an iMac (Retina 5K, 27-inch, Late 2014).I am running OS X El Capitan Version 10.11.4. I am having an issue connecting another monitor with my mac. The current set-up I have is as follows:

iMac using mini display port to HDMI adapter going into my mac and connecting the other end of the HDMI into another monitor. And because of this i have two screens. I have my primary mac screen and secondary screen.

However I am now trying to connect a third monitor. I have a studio and the third monitor will go inside the recording booth. But this monitor does not have an HDMI port. It uses a DisplayPort. So i have bought an HDMI to display port adapter. The other end of the third monitor HDMI cable is going into an HDMI Splitter box. The second monitor going into the Splitter box as my second output and the iMac going in to the splitter box as my Main Input.

The purpose of this is to have the third monitor mirror what the second monitor is showing, While the iMac is the main screen.

But the third monitor, inside the recording booth does not display the screen and continues to say 'check video cable.' So I was wondering. Have I wired it wrong? Is there something I need to adjust in the iMac settings? Could it be the HDMI to DisplayPort adapter? Should I give up and try a VGA cable connection to the second screen instead?

Thanks for the help.

-Hamal S.