I have full custody of my two nephews someone else has claimed them now how do I do my taxes they have been with me for the last 3 yrs

If you have full custody, then nobody else should be able to claim them. Being you have custody and they live under your roof, they're your dependants. First I'd question the court who granted you custody. Then I would also question whoever prepares your taxes, if you do them yourself, look into it at www.irs.gov and do a search on it there.