Time Traveler

I run to the west, escaping immediate danger, but I am being pursued. Immediately I come to the bank of the Enipeus River. I can't go back. I recall that Caesar's cipher simply substituted each letter in the original message, the plaintext, with the letter three places farther down the alphabet. A would be written as D and so forth. But I still do not have time to translate the message. The soldiers are almost upon me.