A surface-acoustic wave (SAW) creates its own high-Q ultrasmall-volume whispering-gallery mode (WGM), different from the usual bulk acoustic WGMs, in an optical dielectric WGM resonator. We show that it is possible to realize an externally controllable, efficient triply resonant optomechanical interaction between two optical WGMs and the SAW WGM and to use such an interaction in various sensor applications.