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!

GU FRC OZKETRFU TLQ ULP VOI YCOQ FREM KU DYECIQ E OK EKNYCMMCQ ACYU HCZZ QLIC ULPY MLZPFELI BCU EM IYYOYTGGYLGR FREM ZEFFZC VROZZCITC HOM ILF FLL ROYQ HOM EF