The polarization transform errors of three-waveplate polarization transformers (TWPTs), which consist of one half-waveplate (λ/2) and two quarter-waveplates (λ/4), are discussed detail. The error models of the TWPT using a new and simple two dimensions geometry method instead of traditional Poincaré sphere method are deduced and numerically calculated. The results of investigation show that the errors of the TWPTs are dependent on the different combinations of one λ/2 and two λ/4, and according to the accuracy of TWPTs, λ/4+λ/2+λ/4 is the best, λ/2+λ/4+λ/4 is better and λ/4+λ/4+λ/2 is good. After polarization transform, the errors of azimuth angle and ellipticity angle are rather large and minor respectively and the TWPTs have enough accuracy to meet the applications in the field of coherent optical communications and optical fiber communication systems.