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!

TM YBP AIFKZBYM ZRD MRG WAU JPAD YBKC FM LJKPUD K AF KFOJPCCPD HPJM NPII DRUP MRGJ CRIGYKRU XPM KC UJJAJZTTJRTB YBKC IKYYIP WBAIIPUZP NAC URY YRR BAJD NAC KY