To say the SR-71 “outran” missiles is incorrect. It would be more accurate to say that it was too high and too fast for the SAM systems of the day.
Think of shooting at geese with arrows. I can shoot at a goose 80 yards away horizontally, or at a goose 100’ up, but I cannot shoot at a goose that is 100’ above a spot 80 yards away. In fact, there is a very small vertical cone that I can shoot at geese flying at 100’. If I’m also not able to see outside of that cone, then I might not even be able to get an arrow to 100’ before the goose it out of range. That is how a Mach 3+ aircraft was immune to Mach 5 missiles.
With modern SAM systems, the SR-71 would not be safe relying on kinematics alone. Not because the missiles are faster, but because the guidance is better and allows predicted intercepts, e.g. AEGIS.
Assuming the 3500kt figure is the plane’s top speed, then:
3500kt = mach 5.29
It might be touch and go, but I think the plane would still out run a Mach 5 missile, also assuming such missile was handy.