In 1/25 scale, 1 inch is exactly .040 inch ( or approximately 1 millimeter). So 3 scale inches is .120 inch.
Years ago, I was told that the difference in a Mustang and Cougar was that the front suspension pieces were swapped side for side. The suspension has a 1-1/2 inch offset from the mounting points. On the Mustang, they angled rearward resulting in a shorter wheelbase, while the Cougar had them angling forward. The guy that told me this was the son of a Ford executive from the 60s (and a well known modeler)the late Randy Vandriass.