Reddit reviews XB-70 Valkyrie: The Ride to Valhalla

According to XB-70 Valkyrie: The Ride To Valhalla, published 1998, AV-3 had a somewhat different design than the first two. A different crew configuration (4 crew total), a radome, a larger equipment bay and weapons bay, and some other changes, mostly in the lower fuselage section (engine pod area). The raw materials had all been procured, and the parts were being fabricated, and some of the avionics were complete. By that time, the writing was on the wall for the program, with SAMs and ICBMs negating its role, and a whole grab-bag of possible uses for AV-3 were proposed in order to keep the program alive. They suggested high-speed aerodynamic research, use as a launch platform for satellites on rockets or satellite-killing missiles, as a test bed for new types of high-speed and space engines, and was even evaluated for the new hotness of low-altitude penetration, which it was manifestly unsuited for.

The book says that in 1997, there was some evidence that parts of the AV-3 might be found in a particular factory in California, but all that was found was a bit of the cockpit section, the rest possibly having been scrapped in the early 1990s. There were only a total of 32 YJ-93-3 engines made for the program, and the book says only 17 could be accounted for, with no records at all indicating where the others might be, or if they had been scrapped.