I have only had full legal custody of my niece for 3 months, can I give her back?

Typically, the authority of a guardian terminates; (1) When a minor ward becomes eighteen years of age; (2) Upon an adjudication that an incapacitated or disabled person has been restored to his capacity or ability; (3) Upon revocation of the letters of the guardian or conservator; (4) Upon the acceptance by the court of the resignation of the guardian or conservator; (5) Upon the death of the ward or protectee except that if there is no person other than the estate of the ward or protectee liable for the funeral and burial expenses of the ward or protectee the guardian or conservator may, with the approval of the court, contract for the funeral and burial of the deceased ward or protectee; (6) Upon the expiration of an order appointing a guardian or conservator ad litem unless the court orders extension of the appointment; (7) Upon an order of court terminating the guardianship or conservatorship. 2. A guardianship or conservatorship may be terminated by court order after such notice as the court may require: (1) If the conservatorship estate is exhausted; (2) If the conservatorship is no longer necessary for any other reason; (3) If the court finds that a parent is fit, suitable and able to assume the duties of guardianship and it is in the best interest of the minor that the guardianship be terminated. 3. Notwithstanding the termination of the authority of a conservator, he shall continue to have such authority as may be necessary to wind up his administration. 4. At any time the guardian, conservator or any person on behalf of the ward or protectee may, individually or jointly with the ward or protectee, or the ward or protectee individually may petition the court to restore the ward or protectee, or to decrease the powers of the guardian or conservator, except that if the court determines that the petition is frivolous, the court may summarily dismiss the petition without hearing. 5. Upon the filing of a joint petition by the guardian or conservator and the ward or protectee, the court, if it finds restoration or modification to be in the best interests of the ward or protectee, may summarily order restoration or modification of the powers of the guardian or conservator without the necessity of notice and hearing. Communication of information by, in, to or through this Web site and your receipt or use of it (1) is not provided in the course of and does not create or constitute an attorney-client relationship, (2) is not intended as a solicitation, (3) is not intended to convey or constitute legal advice, and (4) is not a substitute for obtaining legal advice from a qualified attorney. You should not act upon any such information without first seeking qualified professional counsel on your specific matter.

How did you get legal custody? Through a court order? If so, you can make a motion to the court saying you are unable/unwilling to continue to care for the child. If what you're calling "legal custody" came to you by something signed by a parent, it isn't legal custody and you can return the child to the parent any time or call CPS and say the child has been staying with you and you can no longer care for the child.

It's more complicated than that: What was the reason you got her in the first place?