When you are entering Canada with a still valid PR card, there's always a question if you will get stopped or not. They don't register you leaving, only coming so they might not know if you were gone 3 years or 3 weeks. However, they have their ways of finding out about things and they will definitely not tell you what they are so you don't know what they know. When you are coming back to Canada after 3+ years outside and you fill out that little customs form, you tell them how long you were gone. If you lie, they may find out and use it against you later. They may also ask you at passport control how long you were gone in the form of a friendly chit chat which can also be used to trip you up if you are not careful. If you were in this situation, it might be easier for you to cross the border by land from the US.
If they do catch you on entry and question your PR status, they may decide that you have been outside Canada too long and that you don't get to keep your PR status anymore. Apparently they will then let you into Canada and give you 30 days to appeal their decision, either prove you haven't been outside Canada that long or appeal on humane and compassionate grounds.
If they don't catch you on entry, the next check is when you apply to renew your PR card. At that point, you will have to list your absences from Canada in the previous 5 years. If you lie, like I said before, they may have their ways of finding out. Lying to immigration is not very good when you get caught. If you don't lie, then you have basically brought it to their attention if they didn't know it already that you lost your PR status. Better would be not to renew your PR card until you meet the residency requirements again because if you apply for a new PR card with solid 2 years in Canada that you can prove, they are not allowed to look any further back.
