Tuesday's acquisitions shore up a defense that struggled in the first two games, both losses.
Olshansky has started 98 games in seven seasons, including 14 for Dallas last year, when he had 38 tackles and no sacks. The Cowboys cut him just before this season.
Jones played with Miami in 2008-09 and spent the 2010 season with the Denver Broncos, who released him in August. The eighth-year veteran has 10 career starts.
Johnson, a two-time Pro Bowl running back who sat out most of last season, played only briefly in two weeks with Miami.