It looks like FL allows all 26 letter and 10 digits (at least according to the custom plate availability wizard) and in FL the last 3 positions of the plate include letters and numbers so in FL it would be 1 in (36 * 36 * 36) or 1 in 46,656 to get 986 as the last 3 characters of your plate. Then subtract a handful of disallowed combinations that make bad words, etc
