3 monitor setup with FCP X

I was not aware that you can get full screen output from rMBP's HDMI out. And it seems to work better than my Matrox MX02 Mini. According to Apple the new MacBook Pros also support 4K out of HDMI.
Quite nice!

Please can I ask about this setup - as its what I am trying to achieve. You have one screen for the browser plugged in over Thunderbolt then your playout monitor plugs directly into the MBP's HDMI port - without any third party breakout hardware? (eg. Aja Kona / Blackmagic devices)

Yes. Follow these steps: support.apple.com/kb/PH12783

It's interesting that the Apple support page says that you can monitor "SDR AND HDR" through HDMI. For an iMac, you needed the $2500 AJA Kona something box to get the HDR metadata through to an HDR monitor/TV. Older MBP had HDMI, but the wrong protocol, newer only had Thunderbolt.

But that 1080p AV-out was supported via HDMI had always been the case. Found that out accidentally with my cheesegrater a few years ago, after having bought and installed the Blackmagic Mini Monitor card (which didn't work too well with FCP, although with Resolve).

Let's hope the upcoming new iMacs will feature an HDMI port again.