Harlequins RL have signed wingers Michael Robertson and Liam Colbon and back rower Matt Cook.
Colbon, 26, and Cook, 24, have both joined from Super League rivals Hull KR and have signed two-year deals.
Robertson, 28, began his career in the National Rugby League with Canberra before moving to Manly in 2006, making 147 appearances for the Sea Eagles.
On Robertson, head coach Rob Powell said: "He is a proven try-scorer and is coming in the prime of his career."
Robertson added: "I look forward to enjoying plenty of success at Quins.
"Harlequins RL is a club on the rise and I've heard nothing but great things about their set-up.
"While leaving Manly is a very tough decision, I'm really excited about the next phase of my career.
"I've always wanted to play in England and felt it was important I went over while I was in good form and playing well."
The Australian played in 139 consecutive games for the Sea Eagles between 2006 and 2011.
Colbon started his career at Wigan before moving to Rovers in 2009.
He said: "It's a great opportunity for me to show what I can do and I'm really looking forward to joining up with the squad and to start working with them all in the off-season."
Cook began his professional career with Bradford Bulls and spent time on loan at Castleford prior to joining the Robins.
"I'm really excited to be coming home to Harlequins Rugby League. It's where I started my career when I was a kid and I was coached by Phil Jones," he said.