Eyefinity non-widescreen

While I haven't heard of this specifically, I have heard of people doing the reverse, using a widescreen monitor in the center and flanking it with two 4:3 screens (oriented vertically). If they can do that, I don't see a problem with your attempt, assuming all three monitors have the same vertical pixel count/height (which might entail flipping the orientation of the 4:3 monitor).

Mixing screens with different aspect ratios will work fine in windows on the desktop and such, but the picture will be messed up in games.

In games you have to set your aspect ratio so that your picture is not stretched or squished. If you use a large aspect for a widescreen the picture will look good on that screen but the world will be tall and skinny on the 4:3 monitor. In reverse if you set your aspect to something small for the standard screen the world will be short and fat on the wide sreen monitor.

There are a few tools like widwscreen fixer and soft triple head out in the world that can sort of solve for this but they do not completely fix it.

Unfortanitly AMD eyefinity and Nvidia Surround only trick the computer into thinking your 3 monitors are one big monitor, they dont correct for lack of resolution or aspect or field of view. Its a shame because if they had built in support for mixed screens and a way to correct the aspect on games where its not adjustable then multi monitors for gaming would be a viable option.

So if you just want three screens for productivity you can mix and match any way you want, but if you want eyefinity for gaming all three screens must have the same aspect ratio.

Also keep in mind any game that has a locked aspect ratio or field of view will simply not work right. The picture that was intended for screen just gets stretched across all 3. The CoD modern warfare series for example.

There is a program called wide screen fixer that helps with this but it is one guys project and has limited support for games.

Eyefinity 2.0 which is being released any day now, adds support for multiple resolutions (i.e. 2 24" monitors with a 19" monitor)

I plan to do this myself, so I can't wait to see how well it works (or not).