Centre Marsh and half-back Mellor spent most of last season on loan at Widnes, and are expected to complete permanent moves to the Stobart Stadium following the end of their contracts at the DW Stadium.
Winger King made just one appearance for the Warriors first team in 2012, and also moves on.
Head Coach Shaun Wane said: “I would like to thank Stefan, Joe and Ryan for the hard work and commitment they have shown during their time at Wigan and would like to wish all three the very best for the future.
“It has been decided not to renew these players contracts and to allow them to pursue their Rugby League careers elsewhere. Wigan has a very strong squad with a number of other up and coming young players who are also challenging for places in the First Team for next season and it is felt that the opportunities for these three may be somewhat limited as a result.”
Marsh, who scored seven tries in 20 appearances for Widnes last season, played 12 times for Wigan after making his debut against Huddersfield in May 2010.
He was joined at the Vikings midway through last season by Mellor, who scored four tries in 11 appearances. He also made two appearances for Wigan in 2012, scoring one try, ironically in the 37-36 defeat to Widnes in March.
Mellor made four appearances in total for Wigan, and actually made his Super League bow during a one-game loan spell at Harlequins in 2011.
King made a handful of appearances for South Wales Scorpions on a dual-registration deal this season, and played for the Warriors in the 98-4 Challenge Cup win over North Wales Crusaders back in April.