Subtracting Masks

Guys,
I have 3 parallel nodes masking different parts of the face. In 4th node I want to subtract first 3 nodes selections. Is it possible?
That is an outside node of first 3 nodes.
TIA

Have a look at using a Key Mixer Node (Chapter 111, page 2146 in the Manual)

The Key Mixer is a bit of an "underground" feature that I don't see a lot of people use, but it can be very powerful. There is a free Goat's Eye tutorial on YouTube that covers it pretty well.