In this paper, a series of designs for cavity balanced and unbalanced diplexer are proposed. The balanced and unbalanced designs can be categorized into four groups - unbalanced-to-unbalanced, unbalanced-to-balanced, balanced-to-unbalanced (B2U), and balanced-to-balanced. First, two approaches to achieve out-of-phase characteristics of three fundamental modes, namely TE, TE, and TM in a single triple-mode resonator, are proposed for balun filter designs. Second, four types of unbalanced and balanced diplexers are presented by adopting these three fundamental modes, of which the Butterworth response applies with specific external quality and coupling coefficient. To the authors' best knowledge, full-metal cavity balun diplexer and balanced diplexer are not reported in the open literature. For proof of concept, the design of a B2U diplexer is fabricated and measured. Good matching between simulated and measured results shows the accuracy of the proposed design and methodology, which would be attractive in the high-power radio frequency (RF) front-end systems.