I have a triple screen setup(portrait) and I do get decent frames in Arma at 3240x1920. That being said I have 2x R9 280x SSC and an i7 4790k. It does struggle depending on server performance but its hard to plan for that(Averages Around 30 during big ops may drop to 20 but most of the time its around 40-55 this was also on Nziwago so take it with a grain of salt). I would not recommend doing a triple screen setup with just the one card. The resolution alone will kill your vram. I would double up on the 970s. What other games do you play?
PS To touch on what Cpt Whitby said I can understand the bezel bothering some people. I debezzled my screens (took all the plastic off and overlapped the middle screen slightly in front of the side two. Its about a 3mm bezel between screens) At the end of the day I don't even notice it to be honest. Its like wearing glasses, how often do you notice the frames themselves?