They might be separated if they are difficult to place children- special needs, behavior problems. There is a real likelihood foster homes willing to take three siblings may not be available. If one of the children abused another, they would be separated. If one of the children needed a therapeutic foster home and the others didn't, they could be separated. Sometimes the kids could be separated until a foster home willing to take all three opens up. If the children have a wide age range, infant to teenager, a home might not want infants or not be able to take teens. If the three children were together and one because a behavior problem or had emotional problems, that child might be placed in a higher level of care.
The goal is to keep siblings together, but the reality is that a foster home with three open slots can be rare.
If one turns 18, she would need to be able to show that she could take care of the children financially and emotionally. While foster parents are paid a stipend, most states don't give stipends to kinship care (relatives). She would need a place to live and to demonstrate that she has the maturity and social support to care for her siblings. If she has housing, that's a big part of the situation. Unfortunately most teenagers don't have homes or jobs that can support themselves, let alone siblings.
I need some help with a few things.