Polybius Cipher

12 42 42 21 43 44 25 33 11 21 34 13 25 34 23

Based on the ciphertext you suspect a Polybius cipher, but when you decrypt you get nonsense, so the sender must have used a keyed Polybius cipher. You know that the sender is lazy and suspect that the keyword is short (length 3). By bribing the secretary of the sender you find out that the keyword contains the letter P and the other two letters of the keyword precede P in the alphabet.

Decrypt the message, and describe how you solved the problem.