Third order Intermodulation product (IM3) is an in band spurious frequnecy component which degrades the sensitivity of the desired signal. IIP3 point is a measurement to determine the severity of this effect hence the linearity of the system. The intercept point is when the fundamental component sweep intersects with the third order component sweep. Hope this helps
There is a nice paper describing what do you want to know from Prof. Willy Sansen, "Distortion in Elementary Transistor Circuits", IEEE TCAS2, vol. 46., no. 3, Mar. 1999.
From this paper we can see that, for small signal, weakly nonlinear system and first order approximation, IM2 = 2HD2 and IM3 = 3HD3.
As well as IP3 = Vin/sqrt(IM3) or (IP3)dB = (Vin)dB - (IM3)dB/2
Obviously, the small IM3, the larger in value of IP3.