Short answers, pulled from the story.
Groundbreaking on Madison Square Garden III took place on the 9th of January, 1925, and the arena was completed in 349 days at a cost of $4.75 million. It was built by boxing promoter Tex Rickard, who funded the project through a group of backers he called his "600 millionaires."
The largest crowd in the history of any of the four Madison Square Gardens was 23,190 people, recorded on the 17th of January, 1941, for Fritzie Zivic's welterweight title defense against Henry Armstrong. That figure has never been surpassed at any subsequent Garden.
Jean Ratelle scored the final goal in Madison Square Garden III on the 11th of February, 1968, during a 3-3 tie between the New York Rangers and the Detroit Red Wings. The goal came with 19 minutes and 15 seconds remaining in the third period.
The very first event at the third Madison Square Garden was a six-day bicycle race held from Sunday the 29th of November to Friday the 5th of December, 1925, weeks before the arena's official opening on the 15th of December.
The Ringling Bros. and Barnum & Bailey Circus held scheduling priority over all Garden events, including Rangers playoff games. When the circus was in residence each spring, the Rangers were displaced and sometimes forced to play in other NHL cities. The Rangers' first Stanley Cup championship in 1928 was played entirely at the Montreal Forum because the Garden was occupied by the circus.
MSG III hosted a major boycott-Nazi-Germany rally on the 15th of March, 1937, sponsored by the American Jewish Congress and the Jewish Labor Committee. In February 1939, the pro-Nazi German American Bund held a rally of 20,000 there. In March 1942, the memorial service We Will Never Die drew 40,000 attendees across two performances, honoring the two million Jews known to have been murdered in Axis-occupied Europe.