Xavier McKinney, Tae Crowder and Graham Gano were activated on Saturday in time for Sunday's game against the Bengals, but Oshane Ximines will require season-ending surgery for a torn right rotator cuff.
The moves bolster the defense with a pair of rookies who have been on injured reserve.
McKinney, a safety and second-round pick, will make his NFL debut on Sunday. He suffered a fractured foot late in training camp and required surgery.
Crowder, a seventh-round pick, suffered a hamstring injury against Washington last month. That was the same game in which he returned a fumble for a touchdown to give the Giants a fourth-quarter lead in their first victory of the season.