The PR card is valid for 5 years for travel, that is for you to use to get back into Canada after you have travelled. To keep your permanent resident status is another matter. It used to be that you had to live in Canada for 6 months out of every 12 to keep your status. They relaxed those rules to living in Canada for at least 2 years out of every 5. Exceptions if you were out of the country accompanying a Canadian citizen spouse, working for a Canadian company (pretty picky still about what is defined as such) or accompanying a PR spouse who was working for a Canadian company.
When you leave for more than 3 years out of 5, they consider you as having chosen somewhere else as your permanent residence and so they cancel your PR status in Canada. From what I have heard on here, when you arrive in Canada, they will most likely tell you that your PR status is cancelled and let you in for 30 days where you can file an appeal that is almost never successful.
You can talk to an immigration lawyer but I think better for you to apply for PNP if you want to come back. You can get your PR status back within one year.
Stuff about residency requirements for PR: http://www.cic.gc.ca/english/newcomers/about-pr.asp
