What IcE StOrM the warrior said. The one we don't know those 3 symbols in is the Author's one. The other two we know all 26 symbols, because they had complete keys to them. For the Bill code, Q, X, and Z are
, , and respectively. For the combined one, it's , , and respectively. It sounds like the one you're talking about is that last one.
The first one did not confirm these three letters, and they might never be confirmed. My headcanon is that Q is
, X is , and Z is However these aren't official. They're the three symbols also found in the Bill code, as different letters (B, P, and K, respectively).