The theory of three-wave mixing and Thomson scattering in plasmas is re-examined in the low-temperature limit with a kinetic model, giving a more complete description of Thomson scattering in this limit. Errors in the traditional fluid approach to three-wave mixing and scattering are identified and a new corrected fluid approach is given. These corrections to the theory may have important consequences for the analysis of collective Thomson scattering experiments, such as fast ion diagnostics and for the assessment of the feasibility of certain measurements.