May play a role in vesicular transport from endoplasmic reticulum to Golgi. Homodimer. Component of the multisubunit transport protein particle (TRAPP) complex, which includes at least TRAPPC2, TRAPPC2L, TRAPPC3, TRAPPC3L, TRAPPC4, TRAPPC5, TRAPPC8, TRAPPC9, TRAPPC10, TRAPPC11 and TRAPPC12. Heterodimer with TRAPPC6A. The heterodimer TRAPPC3-TRAPPC6A interacts with TRAPPC2L. Heterodimer with TRAPPC6b. The heterodimer TRAPPC6B-TRAPPC3 interacts with TRAPPC1 likely providing a core for TRAPP complex formation. Widely expressed. Expressed in lung, heart, liver, spleen, brain and kidney. Belongs to the TRAPP small subunits family. BET3 subfamily.