Certainly by the mid-1970s the world knew that the SR-71 could go above Mach 3. A page of some of the records set in the SR-71 are [link|http://www.wvi.com/~sr71webmaster/srfact~1.htm|here] (scroll down). I don't know when the early records were declassified. The Wikipedia says that based on its dimensions, inlet temperatures, jet engine design, etc., it's maximum possible speed was [link|http://en.wikipedia.org/wiki/Sr-71|Mach 3.44].

Remember there was a big to-do in 1974 (9/1/74) about it setting a new record for NY to London - 1 hr. 54 min. 56.4 sec. Speed 1,806.95 statute MPH, and a world absolute speed record from 1976 (15/25 km straight course) of 2,193.167 mph (3,529.56 kph). I don't think those numbers were ever classified.

[link|http://www.pbs.org/wgbh/nova/barrier/machines.html|NOVA] has a bit more about the planes. Note than an unmanned [link|http://www.uwyo.edu/news/showrelease.asp?id=1864|X-43A scramjet has gone faster than Mach 5].

Cheers,
Scott.