The Fijians dominated South Africa 22-0 in the final, helped by two tries from Kalione Nasoko.
Fiji has now won five of the last six tournaments in Hong Kong.
Captain Osea Kolinisau acknowledged the successful history but told World Rugby it didn't play a big part in their build-up this week.
"We weren't focusing on doing it three times," Kolinisau said.
"We said that we were going to take it as if we were searching for [our] first title in the series and I am glad that everybody was up to par and everybody did their job."