This regexp:
/^admin$|^[А-Яа-я]{2,20}\s[А-Яа-я]{2,20}\s[А-Яа-я]{2,20}$/
should match this string:
Пупкин Василий Иванович
It does match in JS (tested in Firebug console) and doesn't in PHP, and I cannot understand why.
In JavaScript, strings are always UTF-8. In PHP, however, they are a sequence of bytes. In order to allow PHP's regex engine to handle UTF-8 strings, add the u modifier to the regex.
You can also simplify your regex by removing all the uppercase letter ranges and using the i modifier, however this may be locale-dependent so be sure to test it!
For more on modifiers, check the docs!