Transit for three hours. Do they have to apply for visitor/transit visa?

Hello Don, do your parents hold Canadian passports or Sri Lankan passports? It they hold Canadian passport, the entry permit is not needed. If they hold Sri Lankan passport, the transit entry permit is needed for both trips. They can send the required documents to HK Immigration Department directly to apply for it. The application documents may include: filled application form ID 1003A, recent photograph affixed on ID 1003A, photocopy of the their passports, photocopy of their financial standing proofs, photocopy of flight ticktes, and photocopy of proof of the proposed visit in the next destination after visiting Hong Kong, itinerary, etc. The application usually takes four to six weeks, so you need to apply for it early.