The three-letter maximum limitation is actually entirely arbitrary -- I think it went away back with Windows 95. Momentum is a hard force to counter, though -- it's a very uncommon program that uses more than three letters; I've only seen a small handful, and the only one I can name off the top-of-my-head is SI-Metrix SPICE, which uses, e.g., .sxsch for schematics (...and .sx??? for other file types).
And of course be default Windows "helpfully" hides the extensions anyway... making it all that much easier for someone to inadvertently run a virus.
---Joel