Let F be a global function field over the finite constant field Fq with 3 | q − 1 , and let K/F be a cubic cyclic function fields extension with Galois group G = Gal (K/F) =< σ >. Denote by C(K) and C(K)3 the ideal class group of K and its Sylow 3-subgroup, respectively. Let C(K) G 3 = {[a] ∈ C(K)3| σ[a] = [a]} and C(K) 1−σ 3 = {[a](σ[a]) −1 | [a] ∈ C(K)3}. In this paper, we present a method for computing the 3-rank of the quotient group C(K) G 3 C(K) 1−σ 3 /C(K) 1−σ 3. Specifically, when K is a cubic Kummer extension of Fq(T) , we determine explicitly the key factors t , x1, • • • , xt , and [A1], • • • , [At] in the process of computing the 3-rank of C(K) G 3 C(K) 1−σ 3 /C(K) 1−σ 3. Combining this deterministic algorithm along with the structure of class groups for cubic Kummer function fields, the 3-rank of the Sylow 3-subgroup of C(K) is determined explicitly in this specific case. Examples are given in the last two sections to elucidate our computational method.