Re F111 doing mach 3, Jeff Guinn over on aircrew interview says it can.
My understanding is that a lot of the fast jets are limited by thermal considerations rather than thrust/drag, notably the F15. At those speeds, the engines apparently function as ramjets, so if you keep pumping in fuel, they’ll just keep producing more and more thrust. Problem is, they can’t take the thermal stress. It would be like screwing on a gigantic turbo to a little hatchback engine and turning up the boost to 100 psi. In theory you could probably produce 1000 hp, but the engine will detonate long before that. So, the “it can do mach 3” claims sound to me like some guy down the pub claiming his Sierra Cosworth could do 200 mph if he wired the wastegate shut.