Ireland international Keyes, 25, has Super League experience and also played with Bradford Bulls and London Broncos.
Peachey, 20, made his Super League debut for Castleford in 2019 against Catalans Dragons while Turkey international Aydin, also 20, did likewise with Trinity against Hull FC last term.
All three players come into contention for York’s Championship game against Sheffield Eagles on Sunday.
City Knights head coach James Ford said: “I’m absolutely delighted to bring in Joe.
"I’ve known him for a number of years from games we’ve played against him and that I’ve previewed.
"He brings a great attacking threat and he’ll bring a lot to our performances and preparation.”
On Peachey, he added: “Lewis is an uncompromising player who carries intensity in his contact.
“I remember him from my time with England Academy and I remember a good kid who had some promise.
“I’m pleased to be in a position to have a closer look at him."
Meanwhile, before taking on the York job full-time, England Academy coach Ford used to teach Aydin in his previous job.
“With Yusuf, he was in my last form as a teacher (at Wakefield College)," he said.
“He’s always shown a pretty solid attitude towards learning and he is a tough, uncompromising middle who brings some aggression to the team.”