Is there any screen with resolution of 3440x1080 that is supported by a mid 2012 13" MBP?
Intel HD Graphics 4000 supports resolution up to 1920×1200 through HDMI and 2560×1600 through DisplayPort.

Your Mac technically can only support it's own internal display and one external display.

I say technically, since people are creative and here is an example of 3 external monitors.

https://www.youtube.com/watch?v=w8gE-ckBwjE