I have already purchased one of these computers for a friend and when I tried connecting it to my 2560x1440 Display it was re calibrating to 1920x1080 Lenovo said that it should support my displays, but they didn't seem very confident with that answer. It may have something to do with the conversion from Display Port to DVI which apparently wasn't Dual Link, but I'm having a hard time finding a simple converter and all of these troubles were with one display not 3?
Since it sounds like you are using a dongle to convert from DisplayPort to Single-Link DVI, you will not be able to display a resolution greater than 1920x1200. 2560x1440 is 16:9 aspect ratio, so it will scale down to 1920x1080 (16:9) instead of 1920x1200 (16:10).
In terms of being able to output to 3 monitors, I do not believe it will be able to do that either. I believe the Lenovo M700 Tiny only has two physical video ports, so you will at most be able to connect 2 displays.