It really depends on how you used it. With a lot of flash uses and reviewing, it can probably last 60-80 shots. If your sister know how to control the use of flash (use when you need it type), set to economy mode, it can last 120-150 shots. I took 70 shots (most without flash) and the battery meter still 2/3 life.
3 batteries not enough? Something is wrong somewhere. Why dont you get a battery pack made with AA battery? I do not know if it is possible but if possible much more feasible.... Of course you do not want to risk it.