Plot-thin 'Three Songs' hits all the wrong notes by Sandy MacDonald
LOWELL -- Writer William Mesnik couldn't be too happy that the movie "A Mighty Wind" -- Christopher Guest's parody of a folkie reunion concert -- came along just in time to steal the thunder from his own small-scale theatrical foray, "Three Songs," now receiving its East Coast premiere at the Merrimack Repertory Theatre. Ultimately, there's not much of a contest.<br /> <font size="2" face="Arial,Helvetica,Geneva,Swis…