Processing of GaN-AlInN-GaN epitaxial trilayers into 3-dimensional microstructures, using a combination of vertical dry etching and lateral wet etching, is discussed. The AlInN layers were grown so as to have an InN mole fraction close to the value of 17% required for lattice matching with GaN. Inductively coupled plasma etching with chlorine-argon gas mixtures was used to define mesa features with near-vertical sidewalls. Refluxing aqueous solutions of nitric acid of 2 molar concentration allowed highly selective lateral etching of the AlInN interlayers exposed on the mesa sidewalls, providing a novel sacrificial layer technology for the Ill-nitride materials. Lateral etch rates of 0.14-0.21 mu m/hr were observed for 100-nm AlInN interlayers. Two distinct applications are discussed. In one example, lateral etching of an AlInN layer was used to expose the underside of epitaxial GaN disks for fabrication of planar microcavities. Here, retention of an optically smooth GaN (000 (1) over bar) surface on the underside of the disks is critical. Microbridges with potential for development as sensors were also demonstrated, and the deformation of these structures provides a sensitive probe of the local strain state of the undercut GaN layer.