I would install (and I did) the Oil Press and Voltz without a doubt. They can be invaluable tools to have, so you can make a better decision than a dummy light.
As far as the 3rd - Oil temp would be decent, but you can pretty much tell when the engine oil hits the 'warm' temp by when you get the pressure drop on the oil press gauge. It is up higher when the engine is cool. As it warms, the oil is lubbed better and gets in all the cracks and crevices, thereby 'reducing' oil pressure to the normal range. Oil temp would only be useful in looking out for an overheat, altough oil temp is usually about 10 mins behind water temp in indicating a problem.
If you have an auto, you may conisder a Trans temp gauge, especially if you drive hard or live in a hilly area.
Air/fuel is probably you best all-around bet for the third gauge. I wouldn't worry about the factory water temp, it seems to do the job ok to me.
I'll be (someday) putting 3 more gauges in the DIN holder I have for under the stereo. When I do, I'll be puttin in Oil temp, trans temp, and an A/F ratio. I already have Oil press and Voltz, and I prob. look at those 2 gauges more than any that came with the car, save the speedometer.
