Foster, 20, Minns, 19, and Duckworth, also 19, follow the recent Broncos signing of George Griffin, with more moves expected to be confirmed soon.
The three Rhinos youngsters are under long-term contracts at Headingley Carnegie, and after making their debuts in 2013, further Super League action with London will benefit all involved, according to Leeds head coach Brian McDermott.
“This is a great opportunity for all three players, the move to London is a chance for them to be regularly involved in a first team environment.
“We hope all three players will have a bright future with the Rhinos and this move is a chance for them have a different life experience, away from home and all the challenges that that brings with it.
“I am sure Alex, Thomas and James will make the most of this chance.”