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Assuming a gain of 1 (0 dB) the slope of the fundamental gain line would be 1:1; the slope of the 3rdorder gain line would be 3:1. The slope of the 3rdorder product line is 2:1 relative to the fundamental gain line. Accordingly, the 3rdorder products increase in power at twice the rate of the input tones and are always three times farther away from the IP3 than the input tones when not near saturation.
The power of the 3rdorder products can be predicted when the IP3 is known, or the IP3 can be predicted when the
relative amplitudes of the 3rdorder tones and the input tones are known.

Here's
a great
link to a paper on intermod frequency prediction for any number of tones.
Cascading IP3 Values in a Chain of Components 
Calculating the cascaded values for IP3 for the system budget requires the following operation based upon ratios for gain and IP3 (do not use decibel values). The standard format for indicating decibel values is to use upper case letters, i.e., OIP3 for units of dBm. The standard format for indicating watt values is to use lower case letters, i.e., oip3 for units of mW or W.

This equation gives the method for calculating cascaded output IP3 (oip3) values based on the oip3 and gain of each stage. When using the formula in a software program or in a spreadsheet, it is more convenience and efficient to calculate each successive cascaded stage with the one preceding it using the following format: 

Conversion of the IP3 and IP2 reference may be accomplished with the following equations: 
IP3_{OUTPUT} = (IP3_{INPUT} + Gain) {dBm} 
or 
IP3_{INPUT} = (IP3_{OUTPUT}  Gain) {dBm} 
The following equation is a series expansion of the mixing (multiplying) of two pure tones: 
Unequal Input Powers
P_{L} = P_{2}  2*(IP3  P_{1})
P_{U} = P_{1}  2*(IP3  P_{2})
Where power units are kept constant in dBm or dBW
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