how to slave sync 3 P.O.'s to OP-1

Trying to slave/sync 3 pocket operators to the OP-1, I can’t figure out which sync modes to use. I got 2 to work just by trial and error (using SY3 and SY4 in that order) but the last unit in the chain won’t sync up. The P.O. manual is no help, I can not discern any specific pattern from the instructions. Anyone else done this successfully?