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!

UM GKS ZHDOEKGM EXF MXL IZW CSZF GKOJ DM VCOSWF O ZD ODTCSJJSF QSCM RSHH FXWS MXLC JXHLGOXW PSM OJ OOCZTJWICVSU GKOJ HOGGHS IKZHHSWES RZJ WXG GXX KZCF RZJ OG