Oh dear, I guess you have cracked the two caesar cryptos...
This one is more difficult. Although a simple substitution is easily cracked...
Again the characters are limited to A-Z... But I think I can come up with a 256 version again.

Enjoy!

XI SBN YMJWABSI AKE IKT HYF GNYE SBWO JI LGWNFE W YJ WJDGNOONE RNGI ZNMM EKFN IKTG OKMTSWKF PNI WO FGGYGAXXGKXB SBWO MWSSMN HBYMMNFAN ZYO FKS SKK BYGE ZYO WS