If you have an amp rated 3000rms at 2ohms and you set the gain to what it should be (~77 volts) and your car voltage stays strong at 14.4 when the sub is playing does that mean you ARE getting the 3000rms out of the amp?
Depends on the signal you're playing and volume setting. The power from the amp is dependant on the signal sent to it. If you turn the volume down or send a signal that is a lower level that the signal the gain was set with, the power from the amp will be lower. Amp output is only constant with a constant signal.
lets say i set the gain on the amp at volume 36 playing a 50hz tone. that would mean if i play the sub at volume 36 playing that 50hz tone and my voltage stayed at 14.4 volts then i would be getting 3000rms from the amp?
Assuming that the same recording level was used on both tones, and the sub was giving you a 2 ohm load, yes. I can pretty much promise you that the sub won't give you exactly a 2 ohm load though.