The Cubs are a rebuilding team looking to cash in on these guys. Happ, Contreras, and Robertson are all going to return valuable prospect capital based on the seasons they are having. They aren't locked up long term and none of their salaries are a detriment.
Sheets, Zavala, and a AAA reliever isn't enough to get even one of those guys by themselves much less all 3 even if you ate Heyward's contract.