Composite Sync is pin 3 on all the Mac DB-15 monitor pin outs I have found. The instruction manual for the Belkin adapter omits some information, namely that DIP switches 5,6, and 7 control monitor sync type. The information is printed on the adapter itself.

H+V Sync: DIP 5,6 ON 7 OFF

Composite Sync: 5,6 OFF 7 ON

Sync on Green: 5,6,7 OFF

VGA: 8 ON (I don't think this changes sync, just forces the sense pins in the adapter to tell the machine to output standard VESA refresh and resolution types)