Help with 3 masked mc under 1 mask

programming flash
1

I start by adding 4 movie clips to the stage.

_root.attachMovie("MC1","mc_1",this.getNextHighestDepth());
_root.attachMovie("MC2","mc_2",this.getNextHighestDepth());
_root.attachMovie("MC3","mc_3",this.getNextHighestDepth());
_root.attachMovie("MC4","mask",this.getNextHighestDepth());

I than want to mask the first 3 movie clips with the 4th one.

mc_1.setMask(mask);
mc_2.setMask(mask);
mc_3.setMask(mask);

However, only 2 of the 3 movie clips are masked.
What’s happening and how can I mask all 3 in AS?