How to distinguish introvert personality, social anxiety disorder and mild autism?
There is an ongoing debate as to whether it is best to view personality disorders as being categorical or dimensional in nature. Your question infers the perspective of the categorical view.  The alternative dimensional perspective suggests that the three disorders you mention are very similar, that is, these three conditions overlap to a large extent with a slight shift along an (as yet unnamed) dimension.
Hopwood, C. J., Kotov, R., Krueger, R. F., Watson, D., Widiger, T. A., Althoff, R. R., Ansell, E. B., Bach, B., Michael Bagby, R., Blais, M. A., Bornovalova, M. A., Chmielewski, M., Cicero, D. C., Conway, C., De Clercq, B., De Fruyt, F., Docherty, A. R., Eaton, N. R., Edens, J. F., … Zimmermann, J. (2018). The time has come for dimensional personality disorder diagnosis. Personality and Mental Health, 12(1), 82–86. https://doi.org/10.1002/pmh.1408