Xiao Cong, Xiao Lin, and Xiao Jun came to the school infirmary at the same time, and there was only one doctor in the infirmary. It takes 3 minutes for Xiao Cong to get an injection, 6 minutes for Xiao Lin to change the gauze, and 2 minutes for Xiao Jun to apply the lotion. To minimize the total waiting time of the three of them, how should they arrange their visits? How long does it take at least?
小聪、小林、小军三人同时来到学校医务室，医务室只有一位医生．小聪打针需要3分钟，小林换纱布需要6分钟，小军擦药水需要2分钟，要使他们三人等候时间的总和最少，应该怎样安排他们的就诊顺序？最少需要多长时间？
<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">6 + 6 + 6</gadget>
<output>18</output>

<result>18</result>