How to distinguish introvert personality, social anxiety disorder and mild autism? There are the following similarities between introvert personality type, Social anxiety disorder and mild autism

Disinterest in social interaction
Interested in objects than people
May be afraid of being centre of attention
Have difficulty in people skills
May lack eye contact
May have special interest and hyperfocus
Have difficulty guessing others' mind

How to distinguish these conditions are due to  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