The issue was that matrox had not give me any 3x(something) option at the beginning.
First I found out that my only HDMI port is connected to my motherboard, not my graphics card (it's a laptop). After that, I managed to create a new custom resolution, 2400x480, the only one that worked, which was a partial solution due to very low resolution. So I ended up with three projectors with 800x480 each.
What I learned so far is that my laptop is not suitable for matrox.