I recommend that you have each one make two different statements.

The problem is that in logic, the statement "A and B" is false if A is false, or if B is false, even if the other one is true.

On that basis, one solution is that Iron is the trickster, Silver is the truthteller, and Brass is the liar with the key. The complete statement "I have the key and Silver is a trickster" is a lie, even though half of it is true.

The simple fix is to make them separate statements:
Brass: “I have the key. Silver is a liar”
Silver: “I don’t have the key. Iron is a trickster”
Iron: “I don’t have the key. Brass is a liar”