------
## 最终结论
```python
import re

def file_name_check(file_name):
    # Check if the file name has exactly one dot
    if file_name.count('.') != 1:
        return 'No'
    
    # Split the file name into two parts
    before_dot, after_dot = file_name.split('.')
    
    # Check if the substring before the dot is not empty and starts with a letter
    if not before_dot or not re.match(r'^[a-zA-Z]', before_dot):
        return 'No'
    
    # Check if there are more than three digits in the file name
    if len(re.findall(r'\d', file_name)) > 3:
        return 'No'
    
    # Check if the substring after the dot is one of the allowed extensions
    if after_dot not in ['txt', 'exe', 'dll']:
        return 'No'
    
    return 'Yes'
```