This should work:

^(?! )(?!.* )(?!.* $)[^\s].*$

The following is a breakdown of the expression:

• ^: Assert the beginning of the line.
• (?! ): Negates lookahead and prohibits spaces at the beginning of the line.
• (?!.*): Negative lookahead, two or more consecutive spaces are not allowed in the string.
• (?!.* $): Negates lookahead and does not allow spaces at the end of the line.
• [^\s]: Matches any non-whitespace characters.
• .*: Matches any character (except newline) 0 or more times.
• $: Assert the end of line.

I ran a small test onregex101.com.