Safety Michael Stone, who will replace Harrison, offensive lineman Gene Mruczkowski and running back Amos Zereoue all were free agents.
Zereoue played in 15 games last season with the Oakland Raiders, rushing for 425 yards and three touchdowns on 112 carries. He is insurance for back-up Kevin Faulk, who suffered a foot injury in the Patriots' games with Pittsburgh last Sunday and lost two fumbles in the contest.
Zereoue was chosen by Pittsburgh in the third round of the 1999 NFL draft out of West Virginia, and has played in 81 games in his career, rushing for 2,123 yards and 10 touchdowns on 546 carries. He also has 136 pass receptions for 1,106 yards and one score.
Stone, 27, was a second-round pick by Arizona in 2001 and has played in 37 games.
Mruczkowski, 25, was originally signed by the Patriots as an undrafted free agent in 2003. He played in 10 games, all with the Patriots in 2004, serving as the backup center and on special teams.