极限 = 3－－－－－－－－－－－－－解析：A = lim(3^n)^(1/n) = 3B = lim(1+2^n+3^n)^(1/n) C = lim(3^n+3^n+3^n)^(1/n) = lim 3^[(n+1)/n] = 3因为 A ≤ B ≤ C，且 A = C = 3，所以 B = 3
