Is it necessary to say Dua-e-Kanoot in 3rd Witr or if somebody forgets? No, necessary in the sense of ordinary terms. Anything which Rasool-e-Karim sallallahu alaihi wa sallam has advised becomes necessary for every Muslim. But in the terms of jurisprudence, it does not fall under the category of wajib or farz. Because the definition of wajib is this, that if you have not said that thing which is wajib, then you have to repeat that part or your prayer will not be considered whole. And say Sajda Sahab as well.
So about Dua-e-Kanoot, the edict of all jurisprudence scholars is this, that if you do not say Dua-e-Kanoot, or at least majority, I can’t say all, I don’t know, the edict of every other jurisprudence, but I know the fundamental jurisprudence believe that if you have not said Dua-e-Kanoot, still your prayer will remain acceptable.
And it will not be faulted to the extent that it should be repeated or Sajda Sahab should be said. So necessary in the sense that even the smallest injunction which has been given issued by becomes necessary for every Muslim to follow. But in religious terminology, in the terminology of jurisprudence, Dua-e-Kanoot cannot be declared wajib or farz.
