Are you sure you're not thinking of the Mk4? Mk3 is an iterative step up from Mk1, new engine, turret, and fcs, but same thin armor and same gun. By the time it was being marketed most countries were moving on to newer better things, Mk3 was meant for poorer countries and therefore only saw purchases in Kenya and Nigeria, where it eventually saw its combat debut against Boko Haram decades after entering service. I can't imagine the Vickers Mk3 would fare particularly well in the Fulda Gap against the latest Soviet armor.