A: Dear Mott669:
Your boyfriend's 3 year old daughter is naturally loyal to her own mother and to her mother's way of doing things. It is not unusual for 3 year olds to sleep in their mother's beds, especially if we look at families all over the world. So this little girl is only insisting on things as she is used to them. And also, it must be hard for her to understand why Mommy and Daddy are not together, and how she can control the world of grown-ups so that things seem more predictible. Her feeling "scared" at night may reflect her insecurity about the important people in her life, and her anxieties about what has happened to her family and about who is going to take care of her. The visits with her mother are important on one hand, but further disrupt the little girl's routine. So she must have many questions about what it all means, which are hard for someone of 3 years to sort out properly.
For these reasons, sleeping in your bed may be the only solution for a time. You can help her gradually transition to her own bed and her own room by putting her to bed and promising to stay with her (sitting quietly nearby and telling her kindly that it is time to sleep, not time for conversation) until she falls asleep. Then you can sneak out of the room. If she is worried and frightened, her fright will keep her alert and wakeful--then she'll never get to sleep! So given the overall situation, you will have to "go with it" and try to be as soothing as you can, until she is more comfortable. This may take a while.
Perhaps you can cheerfully suggest that her mother purchase a little cot so that her daughter can sleep near her, but not in the same bed. But her mother may not want to make this change, which is her right.