I'm an old guy, and was into Classic Mustangs when I was younger (yes, classic when I had it). I pronounce it MACH 3, like the Mach 1. I thought that was how it was meant to be said, and considered mark 3 to be coming from the uneducated car people to begin with. I hear MK3, mark3, and mach 3 all about the same as each other. Mark 3 probably a little more than the others.
I have always had a dream of building a "Baby Mach". That would be an old Celica (one of the first ones sold in America). Have you guys ever noticed the similarities between those cars and an old Mach 1, it's incredible. The Celica looked like a 1/2 scale Mach 1. Check it out! I want to build one with a 7m-gte, and paint it like a Boss 302, even with a faux shaker hood scoop. Muscle car guys would laugh, until it kicked their ass.