christine, i think you are a little mixed up.
a PR DOES NOT need a visa, of ANY kind. so, it is impossible that if they are gone for more than 3 years their VISA will become null and void... they DO NOT have a visa.
what happens to a PR (ONLY those who have "earned" it ~ie. been here for 7 years) is that they no longer have Right of Abode. They are downgraded to Right to Land. They basically have the right to live and work in HK WITHOUT a visa, however, they may get deported if they take up illegal activities.