Of the three players, Legedu Naanee is probably the most well known. He was originally a fifth-round draft pick by the San Diego Chargers in 2007, logging several years of service under then head coach Norv Turner. Naanee's best season came when he left San Diego, though: in 2011, he started 10 games and caught 44 passes for 467 yards and 1 touchdown as a member of the Carolina Panthers. He then signed a one-year deal with the Dolphins in 2012, but was cut a few games into the regular season.
Seyi Ajirotutu also has ties to Turner, as he made the Chargers as an undrafted free agent in 2010, catching 13 passes for 262 yards and 2 touchdowns. Strangely enough, he followed Naanee to the Panthers in 2011, before re-joining the Chargers mid-way through 2012. That gives both Naanee and Ajirotutu double connections to Turner and current Browns head coach Rob Chudzinski, who was formerly the Panthers' offensive coordinator.
Chris Rucker was a sixth-round draft pick by the Indianapolis Colts in 2011. He started four games at cornerback during that dismal Peyton Manning-less season, and registered 36 tackles and 2 passes defended on the season. He was with the Colts during training camp in 2012, but did not make the team's final roster.
We'll see if the Browns decide to sign any of these three players to a contract, or are just using them (particular the receivers) for their familiarity with certain schemes.