I'm wondering what three to get. I'm planning on a turbo application.

You'd probably want boost for sure to monitor how much boost you're running and to know if you're having any boost leaks and stuff.

Also, E.G.T. is probably a good idea so that you can know if you're running lean. The thing about E.G.T. gauges is that I hear that sometimes once the gauge reads that your exhaust is too hot and you're running lean, the permanent damage to the engine has already been done. Therefore, some people recommend a quality air/fuel ratio gauge instead. But if you get air/fuel ratio, be sure to get something with a wideband sensor. Otherwise an air/fuel ratio gauge is not very useful, as the voltage readings from the sensor aren't linear and aren't very accurate. However, air/fuel ratio gauges with wideband sensors are typically very expensive.

Also, I'd get an oil pressure gauge, so that you can monitor your oil pressure and know immediately if an oil return line comes loose or something.

That's what I'd do if I had a turbocharged car. Hope this helps. >>

Yes, yes it does.

Boost, A/F & either Oil Pressure, or Oil Temp.

But now after reading Coom's post i gotta study A/F gauges or look at an EGT.