Arts Collaborative Medford is hosting its third Jam Down By Riverside, a direct descendent of the now defunct blues jam at the Porch Southern Fare & Juke Joint, which was a descendant of the Johnny D’s blues jam. Presented by the Medford nonprofit Down By Riverside, Inc., the Jam is for building community and building skills. People of all ages, abilities, styles and instruments are welcome to join in the jamming. There is not a strict blues-only jam rule, but…