I looked into it a little further... You can group displays with like resolutions as a single display and can use monitors with other resolutions as "extended" displays.
So, if you have 3 monitors running 1080p (1920x1080), you can group them all as a single display. If you have a monitor with a different resolution, it can be used as an "extended" group and used similarly to a multi-monitor setup without Eyefinity.
So running at a lowest common denominator resolution for monitors of different sizes/capabilities and set to the same resolution (1280x1024) all as one group makes sense and seems to be one option if you want all three to be treated as a single display at 3840x1024 through Eyefinity technology. I think the quality on the 23" will be compromised by doing so, though.
Or you could set up the new 19" monitors as one group running at 2560x1024 and use the 23" on its own as an extended group at 1080p. This would seem like the best use of the mixed monitors with Eyefinity. This way, each monitor can operate at its native resolution.
You want all the monitors (the 19s and the 23) as one group if you want them treated as a single screen. You will have to give each monitor the same resolution, then group them. The key is the monitors in the group are seen as a single screen with one resolution. They all have to be the same resolution because when you play games, or when the graphics card renders a single image on multiple screens (as in an Eyefinity group) you can't choose mixed resolutions.
So the 23 inch would have to be included in the group with the 19s. If you had monitors you didn't want to include when gaming but used them for other purposes, they would be part of the "extended" group, similarly to how windows allows you to extend your desktop.
As you know, your situation is not ideal because your dealing with monitors of different aspect ratios. So you're going to have a monitor (your 23") not running in its native 1080p resolution. So, if you use the 23" as your center screen (19-23-19), you'll have to set it to the lower resolution of the 1280x1024 19" monitors and stretch the image on the 23" to fully utilize the entire screen. But in doing so, all of the imagery on your center 23" screen will be somewhat distorted due to the stretch; Windows included.
If you really want Eyefinity with no image distortion, I would consider picking up 1 more 19" if I were in your shoes.