To measure THD you apply a pure sine wave of a particular amplitude and measure the harmonic content in the output signal. Since you put a pure sine wave in, any harmonics must result from nonlinearity in the circuit. However, the THD varies with the amplitude of the signal, so you must give you signal level when you give THD.
IP
3 and CP
1 are also measures of distortion, but they are a bit different than THD. Where THD gives the amount of distortion for a particular signal level, CP
1 is a measure of how big the signal can before it causes a particular level of distortion. IP
3 is somewhat similar in this regard to CP
1, but somewhat indirectly.
SNR is the signal to noise ratio. Or more properly, the maximum achievable signal to noise ratio. It requires that you specify what is an exceptable level of distortion, and then amplitude of the signal that causes that level of distortion and the noise floor. So it combines the two imperfections that limit dynamic range, distortion and noise.
-Ken
You can find more on IP
3 and CP
1 in
http://www.designers-guide.org/Analysis/intercept-point.pdf.