I'm strictly thinking about whether the housing is strong enough to withstand the force by using two fasteners. If there are two fasteners and there should be three fasteners, it's possible the extra fastener is there for the purpose of long term durability so the gasket does not fail at 90 degrees from the fasteners.

First, I'm assuming that there are three fasteners. Can anybody confirm?

Second, if it lasts over to Wal-Mart with 15psi, it will probably go to Miami and back. The gasket will eventually fail under the third (missing) fastener.