Hi Sally,

all are are very similiar and mean 'sign up', but are used in a different context / with digferent collocations:

 "anmelden" is the most general and can be used in many contexts, für einen Kurs anmelden, einen Wohnsitz anmelden, etc.

"melden" is more specific and is used in context with (local) authorities, eg bei der Polizei melden, beim Amt melden

"einschreiben" is most used in context with universities and means immaticulate, eg fürs neue Semester einschreiben

Regards, Michael