ffmpeg -i INPUT1 -i INPUT2 -i INPUT3 -filter_complex amix=inputs=3:duration=first:dropout_transition=3 OUTPUT

inputs
The number of inputs. If unspecified, it defaults to 2.//输入的数量，如果没有指明，默认为2.duration
How to determine the end-of-stream.//决定了流的结束longestThe duration of the longest input. (default)//最长输入的持续时间shortestThe duration of the shortest input.//最短输入的持续时间firstThe duration of the first input.//第一个输入的持续时间dropout_transition
The transition time, in seconds, for volume renormalization when an input stream ends. The default value is 2 seconds.
//指一个输入流结束时音量从正常到无声渐止效果，默认为2秒