An early example; while the first Bobbie Jo (as played by Pat Woddell)got to show her singing skills; this really didn't become prevalent until Meredith MacRae took over as Billie Jo in 1966. Soon after, several songs featuring the actresses playing the sisters began to be produced.
Steve Elliott, as portrayed by actor/singer Mike Minor, often got musical numbers as well.
The Other Darrin: Of the three main girls; Betty Jo (played by Linda Kaye) was the only character whose actress remained the same throughout the show's run.
Romance on the Set: Linda Kaye Henning (who played Betty Jo) and Mike Minor (Steve Elliott) ended up marrying in real life in 1968, several months after their characters tied the knot. The two, however, ended up divorcing by 1973.