Use
ffmpeg -y -i original1.mp4 -i music1.mp3 -i music2.mp3 -i music3.mp3
-filter_complex "[1]adelay=5000|5000[aud1];[2]adelay=10000|10000[aud2];
[3]adelay=120000|120000[aud3];[aud1][aud2][aud3]amix=3,apad[a];
[0:a][a]amerge[a]"
-map 0:v -map "[a]" -c:v copy -ac 2 originalMovie1.mp4