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!

QY RLO FVKHTLRY TXG YXB SFU NOFG RLHW KY ZNHOUG H FK HKJNOWWOG AONY COVV GXUO YXBN WXVBRHXU POY HW UNNFNTQQNXQL RLHW VHRRVO SLFVVOUTO CFW UXR RXX LFNG CFW HR