Northern Ireland international Capaldi, 28, can play either left-back or left midfield and will not be cup-tied.
Martin provides added cover for injured keeper Shane Higgs.
Ephraim has scored twice in seven games for QPR this season, both in the Carling Cup.
"Hogan is quick and direct," Leeds manager Simon Grayson told the club's website. "He'll give us options and competition for places."
Ephraim started his career at West Ham before making his first competitive start while on loan at Colchester.
He then moved on loan to QPR before making the move to Loftus Road a permanent one in January 2008.
Capaldi began his career at Birmingham City and enjoyed a loan period with Hereford United.
But he moved from Blues in April 2003 to Plymouth Argyle, where he won most of his 22 international caps.
Since joining Cardiff in 2007, he has made 68 appearances for the Bluebirds, including nine this season, but had his opportunities restricted last term with a knee injury.
Martin has not played a competitive match for Liverpool but played 25 times as a teenager for MK Dons before his move to Anfield in 2006.
Since then he has enjoyed loans at Accrington Stanley and Leicester City. And he was again out on loan at Tranmere Rovers earlier this month prior to his latest move.