对于A,当 a 1 = m = 时, a 2 = , a 3 = a 2 -1= , a 4 =4, a 5 =3,因此选项A正确.对于B,当 a 3 =2时,若 a 2 >1,则 a 3 = a 2 -1=2, a 2 =3, 或 由此解得 m =4或 m = ;若0< a 2 ≤1,则 a 3 = =2, a 2 = , 或 由此解得 m = ,因此 m 的可能值是 , ,4,选项B正确.对于C,当 m = 时, a 1 = , a 2 = -1, a 3 = +1, a 4 = , a 5 = -1, a 6 = +1,…,此时数列{ a n }是以3为周期的数列,因此选项C正确.综上所述,故选D