Hello Adam,
SAIL does use fixed length 4 digit extensions, however, there is a simple solution to your probem using Aliases. Aliases are NOT fixed length. So, let's say, for example, that your 3-digit extensions start at 200 (many do). You can start Sail at (for example) 5200. Once you have defined your extensions as four digits, the last three will now mirror your originals (200 will be 5200, 201 will be 5201 etc). Now, in Globals, set "Auto create alias digits" to 3. Next, go to the Aliases panel and click "Auto Create Aliases". Sail will automatically generate and link 3 digit aliases to all of your 4 digit extensions.
That's it. Your users can now use their old 3-digit numbers just as before. They can also use the underlying 4 digit numbers if they wish. Also, because SAIL regards Aliases as virtual extensions, you can do all of your switching and grouping referring to the three digit aliases rather than the 4 digit extensions.
Kind Regards
S