The trio have been told they are unlikely to be involved in the squad when they return ahead of the new Sky Bet Championship season, the PA news agency understands.
They are due to be part of the rebuilding job at Carrow Road with the Canaries having already made six signings since dropping out of the Premier League, including Kieran Dowell from Everton.
Drmic has been an expensive failure after arriving on a free transfer from Borussia Monchengladbach last summer.
The striker scored just three goals across 25 appearances and was one of the club's top earners, marking a wretched transfer window.
Expensive loan moves for Schalke goalkeeper Ralf Fahrmann, Sevilla midfielder Ibrahim Amadou and Patrick Roberts from Manchester City were cut short mid-season as Norwich finished bottom of the table.
Drmic was sent off in his last game, a 2-0 defeat against Burnley, and all five of his starts came after the restart following the coronavirus lockdown.
Trybull was one of boss Daniel Farke's first signings, joining the Canaries in 2017, and helped them win the Championship title in 2019.
He started the first four games of the club's doomed Premier League campaign before suffering an ankle injury and was then restricted to just 10 starts after returning in November.
Leitner, who initially joined on loan from Augsburg in 2018 before making a permanent move, played just 10 times last term and failed to make a top flight appearance after October.
He was ruled out of the run-in in July with a hernia problem which needed surgery.
His agent, Wassily Krastanas, told PA: "Together with the club we are looking for a good solution, with which both the club and the player are happy.
"Moritz had a great time in Norwich and has developed in every way. The discussions were and are always open and fair, that is what characterises the people responsible for the Canaries."
The club remains in a strong position as they signed new deals last year.