So far we've looked at catchers, 1st basemen and 2nd basemen. 1st and 2nd base appear to be very weak for the 2012 Cubs and I can tell you before you read further that 3rd base is no different. The Cubs lost Aramis Ramirez to free agency who had been at 3rd since midway through the 2003 season. Before we move on to the projections for next year, let's take a look at what the Cubs won't be replacing.
With the exception of 2010, Ramirez was a very good performer for the Cubs. He was also remarkably consistent. In his half season with the Cubs in 2003 he had a .342 wOBA. He followed that with wOBA's of .396, .390, .381, .385, .384, .392, .321, and .373. With the exception of 2010, Ramirez consistently had a wOBA of about .380. Cubs fans like to think that Ramirez missed a lot of playing time, but that's not true. He averaged 136 games per season from 2003 through 2011. Take away 2009 when he dislocated his shoulder and he average more than 140 games per season. He'd probably have played more if the Cubs had considered moving him to 1st base. He hasn't been a good defender for a long time and I'm surprised the Brewers are even going to play Ramirez at 3rd.
Replacing the big shoes of Ramirez is Ian Stewart who was acquired by the Cubs for DJ LeMahieu and Tyler Colvin. Things didn't look as bad for Stewart as they do now when he first came up. He was a top prospect and reached the big leagues at the age of 22. From 2008 through 2010 he was roughly league average at the plate over 1236 plate appearances. It got real ugly last year when he managed an OPS+ of just 21.
Despite his numbers being average from 2008 through 2010, about 78% of his plate appearances have been vs righties. If he's going to play every day those numbers will be somewhat higher than that and as a result, his numbers will suffer. That's not a good thing because he's not expected to be very good in the first place.
I've used Jeff Baker as a back-up at 1st base because of how well he hits lefties and I've also used him for a few more PA at 2nd base. Baker can't play everywhere and he's another platoon option here. He can't platoon at both positions, but I don't really see anyone else on the roster other than him and Adrian Cardenas who would be a back-up. So until we learn more about who is backing up what position, I'm going to leave the back-up here blank. Stewart's projected batting WAR over 450 plate appearances would be 0.9 WAR. You can increase or decrease that based on your thoughts of his defensive skills.