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!

HW CSX DFGYQSCW QJM WJT PDB IXDM CSYK GW NIYXBM Y DG YGUIXKKXM EXIW OXFF MJBX WJTI KJFTCYJB RXW YK BIIDIQHHIJHS CSYK FYCCFX PSDFFXBQX ODK BJC CJJ SDIM ODK YC