It is a timely boost for Saints, who have lost the five Super League Grand Finals, ahead of their play-off game against Warrington on Saturday.
Chairman Eamonn McManus said: “It’s great to go into the Play Offs with three key signings for next year settled. Paul Wellens is playing as well as ever and is the ultimate ambassador for the Club.
“Likewise, Francis Meli is on top of his game and provides real experience in an otherwise youthful backline.
“Paul Clough is a real Trojan and it’s good to sign him through to his Testimonial season in 2014/15.”
Hometown boy Wellens, 32, is a one-club man, and has made 433 appearances for the club, scoring more than 230 tries.
Clough, 24, is another hometown lad who is a product of the successful Saints academy system, and has made 169 first team appearances since making his bow against Hull in 2005.