Doctor, physician & medical practitioner

in general, these three terms are interchangeably used
they all can refer to those ppl with a medical degree and licensed to practise medicine
be precise, there are some minor differences:
doctor means someone who has a medical degree and licensed
if you are the patient, you also call somebody "doctor" (but not physician or medical practitioner)
and you also call those who possess a PhD or other doctoral degree
physician, to be specific, means somebody working in the field of internal medicine, usually somebody who finished his/her specialist training (6-year postgrad)
sometimes it can also refer to other specialists who prescribe drugs (e.g. paediatrians)
but you rarely, if ever, call a surgeon or an orthopedics surgeon or even a gynecologist a physician
medical practitioner just simply means a medical doctor, somebody who practise medicine (and hopefully licensed)
just a more proper term, it is usually used in formal documents
it specifically means doctors, and not nurse, or other allied health specialist
on the other hand, at some circumstances, it can also mean those working in the field of Chinese medicine, if specified so

doctor是俗稱, physician才是在醫院或醫務所的醫生的正確名稱
因為doctor同時可以解作博士。
而medical practitioner是指從事醫療業的人員