To add silence at the beginning when ffmpeg combines video and audio

On Android, ffmpeg, run the following command to combine audio into the video:

ffmpeg-hide_banner-y-imovie.mp4-ibgm.aac-vcodec copy-acodec copy-bsf:aac_adtstoascout.mp4

At that time, I want the sound to start after 3 seconds from the beginning of the video, but I don't know how to do it.From the beginning, you should prepare a 3-second silence at the beginning of the audio, but depending on the conditions, I would like to avoid preparing two types of silence.

I look forward to your kind cooperation.