The Chicago Cubs and pitcher Ryan Dempster agreed to a three-year contract extension on Saturday, keeping him with the club through the 2008 season. Financial terms were not disclosed. The 28-year-old Dempster owns a 5-3 record with a 3.13 earned run average in 63 games this season. Dempster, a right-hander, also converted 33 of his 35 save opportunities. Dempster, who has not surrendered a run in his last 21 1/3 innings, signed with the Cubs as a free agent in 2004. He owns a 56-59 mark with 35 saves and a 4.83 ERA in 247 career games (162 starts).