you can go back to canada whenever u want as long as ur work permit and visa is still valid, but u will be wasting the limited 3 years time if you are not working
to extend a PGWP you either need an employer who will get a LMIA for you and extend your work permit as a closed permit tied to the employer, OR you have to have a application for PR submitted and less than 4 months left on ur post grad work permit and u can apply for bridging open work permit which will extend ur post grad permit by 1 year and not tied to any employer
good luck