My 3 week old and i have resistant thrush. he's normal weight. trying to get treated but is it possible for baby to get invasive candida from thrush?

Possible, unlikely: Most incidents of invasive Candida in newborns have been in those on prolonged antibiotics, very low birth weight (<1500g) with indwelling catheters or iv's. In addition to nystatin, oral fluconazole is well tolerated and usually has good results. Consider using oral probiotic drops to provide a source of friendly bacteria to compete with Candida.