Two-agent scheduling problems have been widely studied for many years. However, in the real world, there might be many agents competing for limited resources. This study explores a three-agent scheduling problem. The objective is to minimize the total tardiness of jobs from agent 1 with the restriction that all the jobs from agent 2 must be completed within a common due window, and each job from agent 3 needs to be completed within its individual due window. A simple genetic algorithm is proposed to observe the properties of this problem. Computational results show that the proposed algorithm fits the three-agent scheduling problem well.