The function returns 'one' if the input is 'hello' or 'three', and 'zero' for all other inputs.

[CODE]:
```python
def fx(input_str):
    if input_str == 'hello' or input_str == 'three':
        return 'one'
    else:
        return 'zero'
```

Please let me know if you'd like me to run more tests.