Brissett appeared in 19 games with the Raptors last season, averaging 1.9 points in limited minutes.
Ellenson, 23, saw action in five contests for the Brooklyn Nets in 2019-20, scoring just two total points. He has spent most of his career with the Detroit Pistons, averaging 4.0 points and 2.4 rebounds per game over four NBA seasons.
Johnson spent the previous two years with the Indiana Pacers after being selected 50th overall in the 2018 draft. He struggled to find any meaningful playing time, logging just 5.9 points per game in 31 total outings over the two seasons.