Parents sponsor 3 years outside Canada

Bad news: they are at the least technically in breach of the PR residency obligation, thus inadmissible. Odds are, given how long they have been outside Canada, even if their PR cards were still valid, there would be 44(1) reports issued when they arrived at a POE to enter Canada, to be followed by Removal Orders. They can argue H&C grounds. A lawyer could help in that regard (consultation ahead of time; then after in the appeal process if necessary, which seems likely).

If they manage to travel to a Canadian POE, whether or not their PR cards are still valid (say before expiry date this month) or not valid (arrival in May after expiration of current PR cards) they are entitled to enter Canada, as they are still PRs at that point. But, as I said, odds are high they will be issued 44(1) reports and Removal Orders. They can appeal. They can remain in Canada while the appeal is pending. If they lose the appeal, their PR status is revoked.

Once the PR cards have expired, their real problem may be just getting to Canada, unless they carry passports from a visa exempt country. Once the PR card expires they are not supposed to be allowed to board a commercial flight destined for Canada. So, without a currently valid PR card they will not even be allowed to board a plane to come to Canada.