It also deals with the audio aspect.
If I wanted to achieve just the audio effect, I might use:!NoteWorthyComposerClip(2.0,Single)
|Instrument|Name:"Flute"|Patch:73|Trans:0|DynVel:10,30,45,60,75,92,108,127|Pos:10
|MPC|Controller:exp|Style:Absolute|TimeRes:Quarter|SweepRes:5|Pt1:0,64|Pos:7
|Rest|Dur:Whole
|Bar
|MPC|Controller:exp|Style:Linear Sweep|TimeRes:Quarter|SweepRes:4|Pt1:2,64|Pt2:2,32|Pos:7
|Note|Dur:Whole|Pos:1^
|Bar
|Note|Dur:Whole|Pos:1
|MPC|Controller:exp|Style:Absolute|TimeRes:Quarter|SweepRes:5|Pt1:0,64|Pos:7
!NoteWorthyComposerClip-End
But the original question was about how to get a hairpin to start all of the voices doing a Diminuendo together on beat 3
Since a hairpin can only affect one MIDI Channel, all voices to be affected must use that channel.