Can only students doing a 4 year degree course give gre or can a 3 year graduate also give it?

If you have a bachelor's degree or have almost finished getting one, no matter how many years it took, you can take the GRE. The GRE is a test to predict how well you will do in grad school, if your education is sufficient as a base for an advanced degree.

U can take GRE after your bachelors degree only ,no matter three year or four year degree.