Barnsley have taken their options to extend the contracts of Mads Andersen, Jordan Williams and Brad Collins by a year.
All three deals were due to expire in the summer but the Reds had the option to add an extra year.
All three have been regulars in the side which is competing for promotion from League One – currently in sixth place.
Duff said: “It’s the reason you have options in the contract in the club’s favour. They have done well for the club. All three would have suitors if out of contract in the summer. It retains the value in the players. It’s a straightforward thing that happens in football. But it’s good for the club. They have all been great for me this season.
“The club spent a lot of time, energy and hours on these players and they want to protect their assets. It’s good business.”
Duff told the Chronicle: “Jordan probably has grown during the season. He looks after all the fine money which is normally a role for an older player and it’s an important role believe it or not.
"Brad is quiet, the way he goes about his business. I made Mads captain and he’s been excellent.
"In terms of professionalism, I couldn’t say anything bad about them. They are never late, they train pretty much every day. They are always the gym when they need to be. You never have to go looking for them like with some players. They have been great to work with them and I have genuinely enjoyed every minute of working with them.”
Striker James Norwood also has a year extension option on his one-year contact but it is yet to be triggered.
Others out of contract at the end of this season are Luke Thomas, Jack Aitchison, Jack Walton, Clarke Oduor, Jasper Moon, Aiden Marsh and other under 21s players.
Duff admitted he would like to keep Thomas but said he would not be involved in the negotiations.
“There are things that go on without me. That’s the head coach part of it. If players are doing well, the club will do their best hold of them.”