3 hours!? Ye Gods!
Don't do that again, please.
I suppose you wanted to catch star trails on the camera. On film cameras, depending on the location, sky-glow, light pollution, it can be done for a few hours, usually with ISO 100-200 film.
On a digital camera over time you will have noise accumulating on the CCD. The idea is to have the lesser noise possible, but to have the IQ good enough for your purposes. The more you will expose the CCD, the warmer/hoter it will get, and this heat will show up in you picture as noise. Eventually the sensor will saturate and the pic will be a beautiful white rectangle.
The CCD cameras used in astronomy, where usually long exposures are used are cooled using a Peltier element and a fan cooler in order to avoid noise due to heat. Even so, with these cameras (which cost from ~1000USD up to several k USD) that are designed especially for this kind of exposures, one shot doesn't take more than a few tens of minutes. The ulterior image processing is the one that makes these exposures into one beautiful color image like the ones you can see on ESO and Hubble sites.
Now, my advice would be to use ISO 200 or 400 and start in small steps from 1 min., let's say in one minute steps. See at what duration your pic is clear enough - that's up to you - and then start doing several exposures at that duration*. Later you can combine all those pictures into a sort of star trail pic.
You can use noise reduction, but that will usually take longer, as the dark frame needed for this reduction is taken automatically after the shot itself.
You can deactivate noise reduction from the menu and take one dark frame before and one after your shooting session, at the same settings. These dark frames are ideally made with the visor cap and the body lens mount cap on. Later you can combine and average these two dark frames and subtract the result from each of your star trail pictures,
before you combine them.
Good luck for now, and please do not take any more 1-2-3 hour pictures with a DSLR.
LE - *you should start from the widest aperture on your lens and close it by a stop until the vignetting disappears or it is within decent limits for correction with photo software.
Bear in mind that astro photography, starting from the film days, requires a lot of post-processing in order to get decent results.