I don't think any game devs set out to support triplehead (with the possible exception of Overlord), it just happens as a result of good widescreen support and/or hacking by the community.
That's definitely not true. Off the top of my head, here are some titles where the developer clearly gave thought to a good triplehead experience (meaning not only does the game work well in triplehead, but the HUDs are on the center screen, the menus aren't distorted, etc.).
Lock On: Modern Air Combat
Wings Over Vietnam
Source-engined games (i.e. Half Life 2)
rFactor (with a separate viewport for each monitor)
Multiple genres. Multiple titles. Special triplehead features. It doesn't happen by accident, because most titles don't feature these little niceties. Some developers care about it and some don't. The ones who do deserve special recognition.