The paper investigates a semi on-line scheduling problem wherein the largest processing time of jobs done by three uniform machines M1, M2, M3 is known in advance. A speed si (s1=1, s2=r, s3=s, 1≤r≤s) is associated with machine Mi. Our goal is to maximize Cmin-the minimum workload of the three machines. We present a min3 algorithm and prove its competitive ratio is max {r+ 1,(3s+r+ 1)/(1 +r+s)}, with the lower bound being at least max {2,r}. We also claim the competitive ratio of min3 algorithn cannot be improved and is the best possible for 1≤s≤2, r=1.