Pene, If "'" is optional why not? She went to three M.D.'s'offices. "Examples She consulted with three M.D.'s. OR She consulted with three M.D.s. She went to three M.D.s' offices. (plural possessive)
She went to three M.D.'s'offices.
"Examples She consulted with three M.D.'s. OR She consulted with three M.D.s. She went to three M.D.s' offices. (plural possessive)