Casey Mittelstadt is staying in Colorado. The Avalanche signed the veteran forward to a three-year contract, the team announced Tuesday, ensuring the franchise will hang on to its most prized addition from this past season’s trade deadline. The deal is worth $5.75 million per season, a league source confirmed. Since joining the Avs in a March trade with the Buffalo Sabres, Mittelstadt played in 18 regular-season games and made the first Stanley Cup Playoffs appearance of his career.