I don't know of any current planes that go above Mach 3. I'm not even sure if the F-22 goes above Mach 2. Outside of experimental unmanned craft, nothing goes "substantially" faster than Mach 3.
From a military perspective, high cruise reduces range, involving multiple refuelings. It also doesn't help in a head-on approach, but missiles do travel at Mach 3.
The costs of Mach 3 outweigh the benefits unless you have an intercept or recon mission.
So the discussion should be ... "we are here, where should we go next?"
I would say re-examine the X-planes results, look at where we are in materials and CFD, and do another X program.
For example, all modern jets use area ruling. What is the next step in supercritical airfoils for airliners? (The current record is around Mach 0.89 for an airliner at cruise.) NASA has a great track record at basic research.
How we can use fuel efficiently and inject a minimum of toxic gases into the upper atmosphere? The SR-71 burnt most of its exhausts, resulting in no soot, can airline engines do the same?
I can only offer speculation. The subject of mysterious military aircraft is of course a rumor mill and field of conspiracy theories like no other. :-)
https://nationalinterest.org/blog/the-buzz/rumors-secret-war...
It's a combo of this and satellites.
The only credible rumor I believe is that a even stealthier variant of the F-117 was used in the "gap" between the SR-71 and modern drones. But since then almost all ISR is done by drones, with the rare exception when the U2 is still flown.
Otherwise satellites had gotten to the point in the 90s that they could provide real time feeds so it destroyed the need for a high end recon platform like the sr-71 and all other ISR could be done by cheap drones.