Dungeon Defenders Local split screen 3 monitors

Dungeon Defenders was not designed to split the three screens in the way you're looking for. The split screen direction option only applies to the first split for two players. When a third player joins, it always halves the second player's screen in the opposition direction of the first split. Four players always results in the screen in four even parts; at that point it doesn't matter what direction split screen you have set in the options.

A potential solution to your dilemma would be to connect two vertical monitors to eyefinity, making your resolution 2160x1920. You would get the left vertical monitor to yourself, while your friends would get the top and bottom half of the second vertical monitor.

The only other solution I could think of would be to connect four monitors to eyefinity in a square (3840x2160) and have a fourth controller join but not select a character. That way it properly splits the screen into four equal parts (everyone gets their own screen) but only three people actually play.