Tickets for three of the Chicago Dancing Festival’s four free events, which begin August 18, disappeared in a flash today. Two performances at the Harris Theater that went “on sale” at noon were “sold out” by 3:00 PM. Tickets for a lecture/performance to be held at the MCA also were grabbed within a few hours. The final event, at the Pritzker Pavilion August 22, is un-ticketed. Better get there early.