The function f(x) maps an input string x to an output string of the same length. The output string is generated by replacing certain characters in the input string based on a specific rule.

[CODE]:
```python
def f(x):
    output = ''
    for char in x:
        if char.isalpha():
            output += chr(ord(char) + 3)
        else:
            output += char
    return output
```

Note: This description and code approximation are derived from the limited inputs we have tested. Further testing and analysis may be required to ensure the accuracy and completeness of the description.