You can scale it so that you don't have that, but then anything on the 720p display that is out of bounds will not be displayed. Ex) If your 720p monitor was the center monitor, then anything on the outside edges of the monitor would be lost.
VSR can be used to help mitigate that, but multi-monitor mixed VSR settings, and different sizes to boot, can make for a wonky display. A third 1080p display should be super easy to come by on Craigslist for ~$50-75 though, and would offer the best experience for EyeFinity. You could also scale everything down to 720p too.