Mask multiple layers

(no ActionScript used)

When you right click on a layer and specify it as a mask, Flash only masks the one layer immediately below:

To mask additional layers with that same mask, simply drag the additional layers between the mask and the masked layer:

Then, once all desired layers are masked, drag them into the desired order:

Or if you'd prefer, right click on a layer, select "Properties", then select "Masked".

Here's the Flash movie which shows multiple layers being masked by one and only one mask layer:

(this example has no ActionScript)