The picks are expected to join the team at its training facilities in Bradenton, Fla., next week.
Rhyne, a 5-foot-9 midfielder, spent four years at Clemson and started 60 of the team's 77 games. He had 32 goals and 23 assists.
"Rhyne brings a lot of pace and is a player that is very versatile," said RSL coach John Ellinger.
Lookingland played collegiately at Bucknell and was the Patriot League defensive player of the year in 2003 and 2004. Palmer is a 6-3 goalkeeper from the University of Maryland, where he was a four-year starter.
Knowles was on the U-17 New Zealand National Team and also attended the University of Akron, where he started 72 of 74 games.