It was never designed to fly at a sustained M3 like SR-71, just short bursts, then putter along at 1.5-1.8. (lol).
There is no evidence the MiG-25 can fly Mach 3, it is designed to fly all day and every day at mach 2.4... it can manage mach 2.6 for about 20 minutes before having to slow down to mach 2.4, and it can fly at mach 2.83 for about 5 minutes before heat becomes an issue.
To be clear operating at top speeds its flight radius of about 750km would take about 13 minutes in each direction, so it could take off and climb and accelerate to mach 2.6 all the way out to its max flight radius of 750km and launch a missile, and then turn back and fly back to base at Mach 2.4...
The bomber with the Ti Skin was a Valkyrie competitor and was designed for M3 cruise, so Ti skin was needed. Ti is not that hard to fab for framework, with skins and forgings it become more difficult, lots of hinges, fastners, small subframes can be made of Ti. For every kg you save that is more range, payload and efficiency.
Never made it to service and is in a museum... safe from thieves hopefully.
The Russians didn't use Titanium where it was not essential because they made good money selling it on the international market... the Titanium in Soviet Subs was made with Soviet titanium, but so were SR-71s, and now F-35s and Boeing airliners are made with Russian titanium components too.