[0:v][1:v]xfade=transition=fade:duration=1.0:offset=3.0[v1];[v1][2:v]xfade=transition=fade:duration=1.0:offset=6.0[v2];[v2][3:v]xfade=transition=fade:duration=1.0:offset=9.0[v3];[v3][4:v]xfade=transition=fade:duration=1.0:offset=12.0[v4];[v4][5:v]xfade=transition=fade:duration=1.0:offset=15.0[v5];[v5][6:v]xfade=transition=fade:duration=1.0:offset=18.0[v6]