Front-row forward Chris Brooker, a member of England's Under-21 Grand Slam team last year, has joined from Bath, while they have also snapped up Northampton lock James Percival and former Quins academy player Phil Davies.
Davies will return to the Twickenham Stoop from French club Bayonne, with the trio agreeing two-year deals which will keep them at Quins until 2009.
Quins rugby director Dean Richards said: "I am delighted to have secured the services of these three players.
"Ollie Kohn, Will Skinner, Danny Care and David Strettle were relatively unknown when they joined us last summer, but their excellent performances this season have now made them key figures within the team.
"I am confident these new additions to the squad can make a similar contribution."
In addition to the newcomers, Quins have also announced new contracts for props Ceri Jones, Aston Croall and Mark Lambert.