Chaperone protein which promotes assembly of the 20S proteasome. May cooperate with PSMG1-PSMG2 heterodimers to orchestrate the correct assembly of proteasomes. Homodimer. Interacts with PSMG4. Interacts directly with alpha and beta subunits of the 20S proteasome but dissociates before the formation of half-proteasomes, probably upon recruitment of POMP. Belongs to the PSMG3 family.