Sinclair is on loan from Chelsea, Reid from West Ham and Ashton from Fulham. All three have signed until the end of the season.
Sinclair, 19, plays as a striker. Reid, 20, plays at left midfield while 21-year-old Ashton is a left-back.
Palace have let four players leave on loan until the end of the season.
Tony Craig returns to former club Millwall, Ryan Hall moves to Crawley, Jeff Hughes goes to Bristol Rovers and Scott Flinders moves to Blackpool.