We - like many a forum - have had several discussions on the origin and the purpose of the elongated three-minute-markers on chronograph 30-minute subdials otherise marked in 5-minute intervals (confusingly, JohnF has come up with a vintage elongated-4-minute-marker variant).
A number of explanations have been advanced for the elongated three-minute markers, with the need of users (even military ones) to time American long-distance calls measured in three-minute intervals looming quite large.
Looking anew at a Longines A-7 (a U.S. Army Aircorps issued aviation chronograph) from the mid 1930s on MWR (http://www.mwrforum.net/forums/showthread.php?t=22713) and seeing that its 30-minute chronograph subdial is marked ONLY in 3-minute intervals, I am increasingly convinced that the purpose of the elongated three-minute marks is rather basic - its purpose is to make reading the elapsed time easier ... looking quickly at a 30-minute subdial marked with 5-minute intervals only, one may easily be mistaken between, say, 3 and 4 minutes past the interval. Not so with the additional elongated 3-minute markers.
Another way to address the same problem is the 15-minute register as used in the German aircraft clocks of WWII (they switched to 15-minute subdials from 30-minute subdials), and by Breguet for the French Type XX/21 chronographs after the war.
The thesis is open for discussion.