All three sentences, as they stand, are ambiguous.
1. She loves her dog more than her husband.
- She loves her dog more than she loves her husband.
- She loves her dog more than her husband loves her dog.
2. They are flying saucers.
- The people are engaged in the activity of making saucers fly.
- The objects in the sky are UFOs.
3. They are entertaining girls.
- They are doing things so that girls have a good time.
- The girls are employed in the entertainment business.
- The girls are fun to be around.
The only way to tell for certain which meaning is actually meant is either through more explicit phrasing or through context that makes it obvious.
In reality, however, most people would assume the second sentence to have the second meanings—simply because that's the meaning most commonly conveyed. The third sentence is less obvious, as each meaning is used on occasion. And the first depends on how cynical you might be—although some people might think that the first meaning is quite possible.
In almost any actual conversation, or any descriptive passage in a book, there would be the context needed to make the meaning obvious. These kind of sentences very rarely appear completely on their own—either in text or dialogue.