Regex To Allow Space If Followed By Character?

I have an asp.net regularexpressionvalidator that I need to match on a textbox. If there is any text, logically the rules are as follows:

The text must be at least three characters, after any trimming to remove spaces.
Characters allowed are a-zA-Z0-9-' /&.

I'm having major pain trying to construct an expression that will allow a space as the thrid character only if there is a fourth non-space character.

My last attempt was:

^[a-zA-Z0-9-'/\&.](([a-zA-Z0-9-'/\&.][a-zA-Z0-9-' /\&.])|([a-zA-Z0-9-' /\&.][a-zA-Z0-9-'/\&.]))[a-zA-Z0-9-' /\&.]{0,}$

but that does not match on 'a a'.