Limitations of external monitors with MacBook Pro Retina mid 2012

Officially these MBPs can support up to one HDMI display (at 1920x1080 pixels) while simultaneously connected to a Thunderbolt display (at 2560x1600 pixels), OR being connected to two Thunderbolt displays (both at 2560x1600 pixels).

However, unofficially you can exceed this. I'm not aware of anyone who has done exactly what you're wanting to do, but I have come across the following scenarios with other users:

• connecting two 27" Thunderbolt displays each at 2560x1600 pixels while also using the built-in display, with a 23% increase in the MBP's fan speed, but with no long-term damage/problems.
• connecting three external displays while in clamshell mode. However, this can result in the MBP's fans spinning at more than double their usual speed when watching videos etc. Not something I would recommend.
• using a single Thunderbolt display at higher resolutions (e.g. 3440x1440) with the MBP in clamshell mode without any problems whatsoever.
• using two 27" iMacs in Target Display Mode (one using a Thunderbolt connection and the other a Thunderbolt/DisplayPort connection) without any problems (both in and out of clamshell mode).
• using a single Thunderbolt display at higher resolutions (e.g. 3440x1440) while also using the built-in display. This seems to work okay, although the fans will spin about 30% faster. This is probably the limit of how much I'd recommend pushing things.

It's not clear from your question whether you were wanting to use the MBP in clamshell mode, but the info above may give you enough to help you decide whether what you want to do is worth the risk.