Goldenwing has given a good suggestion to try using something other than you finger while training to step up. A few things to keep in mind, birds use their beaks to investigate things and that may be the case with the nibble on your finger depending on the strength of the nibble. As for the actual biting, birds will bite for various reasons, it could be to get you to back off or it could be that she has learned that biting produces an action by you that she desires. If you are feeding her and she is biting she may think that if I bite I will get more food. Three months is still a baby, don't expect too much too soon. Try Goldenwing's suggestion of using a perch for stepping up and if she steps up then offer a small reward. Does she have cuttlebone or mineral block in her cage?