"I'm trying to trigger on 1 channel (the yellow one),"
It should be possible to get some sort of temporary stable trigger on that yellow because Signal 3 always shoots up to Signal 1 giving a long +ve edge.
Perhaps move the trigger level away from the middle Signal 2 level, trigger on AC coupled, or add "Noise Rejection" to the trigger, can't think of much else. 

100%, this is the only edge to target but it must be done with Holdoff to prevent retriggering until the next same edge so a Holdoff of ~550us should work just fine.
Other smaller edges can be targeted in exactly the same way albeit with a more defined trigger level.