Problems with filing status. Divorced, claiming my son as my dependant, ex wife signed form 8332 for me to claim our son 2015,2016 and 2017.Can I File Head of household? Last year she claimed head of house hold even though I claimed my son as dependant, (

I know she plans to again claim head of household, is that her legal right to do so if she signed form 8332 for the three year period?
Any help greatly appreciated.