The cure rate for ALL in young people is very high. It would be very unlikely that they would say "you've only got three years to live" to someone this age, as they would probably be treating to cure the disease altogether.
Most people are cured with chemotherapy. A few go on to receive bone marrow transplants and many of them are cured.
All in all, if you want to have a story about someone who's only got a few years to live, a 15-year old with ALL is not a good example. It's also a very cliched setting these days.
For interest, the treatment is quite complicated. It basically involves induction (very powerful drugs to kill most of the leukemia cells), consolidation (not as powerful, designed to get rid of anything that slipped by), and maintenance (treatment for years, to make sure nothing comes back).
Now, it's not impossible for someone that age to die of ALL, but it would still be unlikely that they'd know at diagnosis she only had three years left.