The 26-year-old Stewart, who had a wrist injury at the end of 2011, batted .156 with no home runs and six RBIs in 48 games. He hit 43 home runs between 2009 and 2010.
"We wouldn't have given up the talent that we did if we didn't think he (would be the starting) third baseman," said Cubs general manager Jed Hoyer, who believes part of Stewart's problem last season was finding a new stance. "We are expecting him to come in and he has to bounce back from last year. We are assuming he does. We are looking at him as our starting third baseman."
Hoyer said the deal was consummated on Tuesday, and the Cubs flew Stewart to Chicago to have the wrist examined by their doctors.
"Currently they said it looks very good," Hoyer said. "He's been hitting off a tee and working out. His wrist injury is cleared up and he's ready to go."
"Ian Stewart is a good defensive third baseman," Hoyer said. "He's a very good athletic player and a gifted third baseman.
"With our first two moves we've attempted to make the team a little less right-handed and also attempted to add better defense. We feel very good with both moves that we've done that."
Weathers underwent Tommy John surgery in 2008 and missed the '09 season. He pitched in Double A in 2010, going 2-2 with a 5.22 ERA in 44 relief appearances. He struck out 48 and walked 48 in 45 2/3 innings. He was added to the 40-man roster after the 2010 season.
"Since (the surgery) he's struggled with his command," Hoyer said. "His velocity is still in the upper 90s. He has a great arm. We hope a change of scenery will help him as well."
Colvin, 26, has been of interest to the Rockies as Colorado scouts feel he is closer to the player who hit 20 home runs in 350 at-bats during the 2010 season as opposed to the one who hit .150 with six homers last season.
LeMahieu was drafted by the Cubs in the second round in 2009. He hit .250 with no home runs and four RBIs in 37 games last season.
Bruce Levine covers baseball for ESPNChicago.com and ESPN 1000.