Jr., Sr., III
Jr. and Sr. are preceded and followed by comma in full name:
John M. Fahey, Jr. (left), went to. . .
but John Jr. hurried. . . .
No commas with Roman numeral attached to name:
Nicholas DeVore III arrived.
For possessive add apostrophe plus s:
John M. Fahey, Jr.'s memo
Joel Bourne III's picture.