The trio are all products of the club’s academy and are now committed to Langtree Park until at least the end of 2018.
Coach Nathan Brown said: “We’re delighted to have signed up Mark, Luke and Greg. They are three good players who are continuously improving the more Super League experience they get.
“Their task now is to keep their heads down, work hard and learn from the experience of playing first grade rugby league. If they do that then they can all go far in the game.”
Centre Percival, 20, made his debut against Leeds in 2013, and has gone on to score six tries in 15 appearances.
Cumbrian prop Richards, 18, has made nine appearances since also making his debut last season, and he has also represented England Under 18s.