Thank you everyone for your helpful suggestions. I gave up on trigger #3 and am triggering the 558.2 amp from trigger #1 and the other two amps from trigger #2. I found a 12v trigger output pass-through in the surrounds amp so I'm triggering the subwoofer amp from it.

Meridian: 861v4 + ID40, HD621, Ultra DAC, 857, 558.2
Speakers: B&W 802D, B&W HTM2D, 2-15" custom subwoofers, B&W LM1
Other: Oppo BDP-83, Mac Mini Roon Core.