It was about travelling thru different dimensions, the girl and her brother were searching for their father. The three 'alien' charecters were Mrs Who, Mrs What and Mrs Which - any ideas as to title/author?
"A Wrinkle In Time" by Madeleine L'Engle. It's still in print, though I don't think many stores carry it in stock. There's a TV movie of it doing the rounds on Sky Movies, though as it's cut down from a Canadian mini-series (itself apparently missing parts of the book), I don't hold out much hope for it's quality.