What is the minimum turning radius of an SR-71?
The Turn Radius of the SR-71 would depend on its speed. The faster it went  the wider its turn radius was. 

The SR-71 had a minimum turning radius at altitude of about 80 nautical miles (NM) . It was not an airframe limitation but a matter of wing area. At 80,000ft, the air is too thin and the wings too small to allow for much lift to turn with.

At a turn radius of 80 NM, the SR-71 would cover about 145 miles, taking about 4 minutes in the process of making a 180 degree turn. 

Details are provided in the SR-71 flight manual handbook, which is now declassified: