The function takes an input string and manipulates only the first character. If the first character is an alphabetic character, it increments it by three positions in the alphabet (wrapping around if needed). Non-alphabetic characters remain unchanged.

[CODE]:
```python
def f(x: str) -> str:
    if x[0].isalpha():
        if x[0].islower():
            new_first_char = chr((((ord(x[0]) - 97) + 3) % 26) + 97)
        else:
            new_first_char = chr((((ord(x[0]) - 65) + 3) % 26) + 65)
        return new_first_char + x[1:]
    else:
        return x
```
Keep in mind that this code only approximates the behavior of the original function.