My 3 year old tells herslef disturbing things when its bed time?

Sounds really sad!! And I'm sure you're a great mother so don't doubt that ... Instead of telling her "not to" say these things, I would try sitting down with her and asking her why she is saying these things? Tell her she will not get into trouble and that she can tell you anything. Maybe prompt her with questions. "Is it because you have to go to bed?" Maybe she thinks if you punish her for something that you don't love her. Sometimes kids just say these things for attention but I would be a little concerned too. Have you noticed any other changes in her moods lately? If things get worse, take her to a child psychologist but I really think that you can talk to her at three.