Here "belly" refers to physical desires. AT: "but they only want to please their own selfish desires" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The words "smooth" and "flattering" mean basically the same thing. Paul is emphasizing how these people are deceiving believers. AT: "By saying things that seem to be good and true" (See: [[rc://en/ta/man/translate/figs-doublet]])