Skull Keys not working in Orul's Crypt

[Poldi]

I got all 3 Skull Keys.
When I use the first one on the lock I hear a click and the key disappears.
But nothing at all happens with the next key.
What did I miss?

[Dr.Disaster]

3 keys -> 3 locks -> one each

[PaladinVito]

You didn't miss anything.

SpoilerShow

As soon as you use all 3 keys, the second boss encounter starts, so be prepared.

[Sir Tawmis]

My problem is - I have all three keys - I have used the two in the one room (with the teleporters to the laboratory and stuff) - but for the life of me, can't recall where I saw the 3rd Skull Key1