I was hoping a staff member would come in here, and you may want to also try the Ask the Staffer thread. In the meantime you'll have to make do with me. What I believe is the situation is that some A319s have 3 masks for 3 seats, other have 4 masks. So you'll never be allowed to have 5 or 6 humans sharing 3 seats (doesn't bear thinking about either!), but 2 adults plus 2 children is allowed in some groups of 3, or indeed in CE.

Each aircraft has its own configuration, and I suspect occasionally individual aircraft have different variations too (e.g. exBMI?). For most A319 my recollection - from an incident I witnessed a few months back, is that row 1 A+C is 4 masks, 1 D+F is 3 masks, then it flips on row 2, so 2A+2C is 3 masks, 2D+F is 4 masks, then the same all the way down.

So I would get your seating set on that basis, however don't be surprised if they have to rejig your seating.

The Airbus safety video shows 4 masks, by way of digression.