Since there's still a space on the line it prevents the regexp from catching the \n{3,} occurrences.

There is no \n{3,} in my code. As for non empty line not getting deleted, that's consistent with what you asked. Are you now asking to consider lines with just whitespace to be empty?

I need to catch all spaces that are preceded or followed by additional whitespace. Instead of just {space}{space} it should also check for {space}\s

That makes no sense. That says that {space}{space} should be collapsed to a space (which happens) and that {space}{newline} should be collapsed to {space} (which contradicts what you did say and makes no sense).