If you are in the small signal region of an amplifier, IP3 will be the same for different power levels. So for instance you can measure IP3 at -10 dBm and use that value to predict the intermods at an input power of -20 dBm, or vice versa (assuming both powers are in the small signal region). If the amplifier is in compression or near it, that is no longer true. The IP3 value measured in compression will vary with input power, so it can not be used to predict intermods at a different power level.