Consider yourself lucky.
You wasted people's time.
You also made a good faith deposit so the owner will rent the apartment to you and to nobody else. This is an enforceable oral agreement.
When you changed your mind, you broke the oral contract. The Owner could have kept all 15K as Liquidated Damages, which is the penalty you suffer in order to be released from your agreement. Theoretically, the Owner could sue you for Specific Performance and force you to pay for an entire years worth of rent.
You are lucky you got away with only losing 3K.
I would have kept all of it because you wasted my time.