I had schengen visa from country A 2015 and another from country B 2016 and refusal from country C 2017. Is it true that if you get a schengen visa refusal ...you must re-apply in that same country that denied the application even if the plans of trip are changed ??
Not true. Whichever embassy you apply at next, they will see all your previous Schengen applications in the system, both successful and unsuccessful ones.
Being refused once does not mean you will have trouble obtaining visa next time. It may be that your documents were not in order when you were refused. If you rectify the mistakes and fulfill the checklist you will be granted visa next time.