The Terps added two recruits to their 2021 class and one to their 2020 class.
Castleberry didn't receive a carry for Michigan last season.
Harrison suffered a season-ending knee injury against Penn State on Sept. 27.
Canada served as the Terps' interim head coach after DJ Durkin was placed on administrative leave.
Durkin was fired as the Terps' head coach in October 2018.
Among other signings, coach Mike Locksley secured five-star wideout Rakim Jarrett.
The Terps flipped local five-star wide receiver Rakim Jarrett from LSU on Wednesday.
Jarrett is the Terps' first five-star recruit since Damian Prince in 2014.