This is a six splits bet, (or 12 numbers).
Track each column hit within each dozen. There are 3 columns within each dozen, there are 3 dozens, this gives us NINE individual lines to track. ok
Trigger: wait until 6 individual dozen/column lines have been hit, then bet the remaining 3 unhit lines.
progression: negative dozen progression (splits) 1 1 2 3 4 6 9 14 21 31 47 70
to make it easier i use notepad and for each new trigger i look at the history of spins:
write down 1,2,3 (these are the 3 columns) then a, b, c, this are the three dozens
so it would look like this:
six hits, (the remaining 3 is bet):
1: a b (first and second dozen is hit within column 1) bet would be c: column 1, dozen 3
2: b (second dozen is hit within column 2) bet would be a & c: column 2, dozen 1 & 3
3: a b c (all dozens is hit within column 3) no bet.
I have tested it playing on dublinbet for 3-4 hours, won totally ~300u, and not a single trigger lost.