The last sentence of subsection (c) of section 403 of this title , subsection (f)(1)(C) of section 403 of this title , and subsections (b)(3)(B), (c)(6)(B), 1 (f)(3)(B), and (g)(6)(B) 1 of section 416 of this title shall not apply in the case of any child with respect to any month referred to therein unless in such month or the third month prior thereto such child was under a disability (as defined in section 423(d) of this title ).