Page "Aerodynamics" Paragraph 59
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According to the theory of aerodynamics, a flow is considered to be compressible if its change in density with respect to pressure is non-zero along a streamline.
The Mach. 3 value is rather arbitrary, but it is used because gas flows with a Mach number below that value demonstrate changes in density with respect to the change in pressure of less than 5 %.
Furthermore, that maximum 5 % density change occurs at the stagnation point of an object immersed in the gas flow and the density changes around the rest of the object will be significantly lower.
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