Encode the following sentence using the Caesar Cipher with a key of 3.
This is a secret message.
Wklv lv d vhfuhw phvvdjh.
Complete the following task based on the instruction and optional input provided.### Instruction:
Encode the following sentence using the Caesar Cipher with a key of 3.

### Input:
This is a secret message.

### Output:
Wklv lv d vhfuhw phvvdjh.