Declassified Flight manual says with commanders Authority Mach 3.3 is authorized. So I would say it can go in excess of that as well.
http://www.sr-71.org/blackbird/manual/5/5-8.php
Just some info the Mach indicator in the YF-12A at WPAFB has a readout at 10 I think it could do Mach5+ not to much over that as even the titanium would melt if exposed to that type of air friction for any length of time.
Plus just look at what the Soviets had for Sams 3 that had better performance envelope then what the SR could do by what the Air Force said it could do
SA-5 Gammon Mach 3.5 and 150,000 ft alt
SA-10 Grumble Mach 4.5 100,000-130.000 Ft alt
SA-12 Gladiator Mach 5 and 150,000 ft alt.
Now if the SR could only do 3.3 and have a service ceiling of 85,000 ft then 1 would of been lost to enemy fire. Its still the only plane I know that got shot at 5000+ times and didn't suffer any damage or hits in the 30+ years of service.
But 3 things would of eventually grounded them even if the US still wanted to fly them
1 the J58 Ramjet was no longer being produced plus spares were also not being made only what was on hand for rebuilds
2 All the dies were destroyed after the last SR-71 was made so spares for the craft for airframes and other sub systems were non available so after the airframes started to show wear and tear they couldn't be replaced
3 JP7 jet fuel it was the only aircraft in the inventory that took this and how it was refined is still classified to this day so it was really quite unfeesable to make for just 1 type of aircraft.