Keppie was already signed until the end of the 2023 season but has been rewarded for a strong season by extending his deal until the end of 2024.
Parker and Garrick have both re-signed until the end of 2023.
"Our squad has performed really well over the last couple of months and begun to show everyone what we are capable of," Sea Eagles CEO Stephen Humphreys said on the club website.
"Stability is an important ingredient of success at the elite level and so I’m thrilled, as I know all Sea Eagles Members will be, that the majority of our squad will continue to work together and with Des over the coming years in realising their full potential.
"I congratulate Sean, Brad and Reuben on their new deals which they have earned through their hard work, persistence and excellent performances.
"They are all perfect clubmen who give 100% each and every time they pull on the famous maroon and white jersey and we love having them at Manly."
This adds to the strong retention by the Sea Eagles recently following the re-signings of Josh Schuster, Haumole Olakau’atu and Toafofoa Sipley earlier in the year.