Ex-South Africa lock Stephan Lewies, Leicester flanker Will Evans and Newcastle second row Glen Young are to join Harlequins in the summer.
Lewies, 27, is on loan at the Lions from the Sharks and made one appearance for the Springboks in 2014.
Evans, 22, has played 11 times for the Tigers this season, scoring one try, and has played for England Under-20s.
Young, 24, is a former Scotland Under-20 forward and has been at Kingston Park since he was in the academy.
Young is the second lock to leave the Premiership's bottom club this week after Exeter announced they had agreed a deal to sign Will Witty.
Quins have not disclosed the length of the contracts that any of the trio have agreed.
They bring Harlequins' tally to seven new signings announced this week for next season following Wales hooker Scott Baldwin, Italy centre Michele Campagnaro and Argentina's Martin Landajo and Santiago Garcia Botta.
"Stephan is an intelligent and hard-working Lock who we know will add real quality at the club," head of rugby Paul Gustard told the club website.
"In speaking to him I was hugely impressed with his temperament and personality and furthermore he has been highly recommended to me by my contacts in South Africa who testify to his character and drive.
"Will is a player I have been fortunate to work with before," added Gustard,
"He is a young man with a very clear goal of where he wants to get to and we are delighted he has chosen to join us as he takes the next step forward in his burgeoning career.
"Glen is an incredibly promising talent and we are looking forward to having him as part of our squad as we look to build on the early momentum we have earned to date,
"He is hungry, energetic and is desperate to improve. He loves line-outs and will add real awareness and acumen to our calling systems and structures."