There is no immigration class that allows students after 3 years to apply for PR.
If you qualify to apply as a skilled worker with at least 1 year skilled work experience in the past and 67 or more points on the self assessment, you can apply under category 3 after a year in Canada even if your occupation is not in the 38 most wanted list and you don't have a job offer.
If you get a post grad work permit and a job after you graduate, you could look into provincial nominee programs and applying as an international graduate. In some programs, you can apply right away with studies and job offer, in others, you would need to work for a little while. If your employer supports your PNP application, you can usually apply right away under PNP employer driven stream.
If you get a post grad work permit and a job after you graduate and work for 1 year, you could qualify to apply under Canadian Experience class.
You can find all your immigration programs here:
http://www.cic.gc.ca/english/immigrate/index.asp