I would go with this very cautiously.
1. When you combine A, B and C to D you got to make sure that the content structure remains the same on D as it was on A B and C. I'm not sure how helpful this can be, but it makes it easier for you to manage each redirect.
The last thing you want to do is do a *.* redirect to C. It has to be as clear and distinct as possible or even thinking from a user's perspective, it would be confusing.
Bottom line is you got to make sure that someone coming into A, B or C following a search phrase to landingpage1 from search engines should lead to landingpage1 on either A, B or C or D.
If I were you, my biggest concern would be to make sure that the user experience is maintained in tact.