Yeah, I think he might have meant ISO-8859-1 or Windows 1252 rather than ASCII; but still, all of those characters would take up two bytes in UTF-8, not 3, unless you used combining diacritics rather than precomposed.
Yes, I meant all of the characters outside of the ASCII range. As in, there are no characters in ISO-8859-1 which take up more than 2 bytes in UTF-8. I guess there are a few in Windows-1252 which take up more than two bytes (like the Euro sign), so it's possible he meant Windows-1252 rather than ASCII.