All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If the matter is as you mentioned in the question that the grandfather died and left three daughters and six nephews (sons of his brother), and all of his parents, wife, brothers and sisters died before him, then his inheritance should be divided on his three daughters, and the six nephews if all of them are full brothers, or all of them are half brothers from father's side only, including the nephew who died after his uncle (i.e. your grandfather). The six sons and daughters of the nephew who died will share the portion of their father.
As regards the daughters of the brothers, they have no share in the inheritance of their uncle.
Therefore, the daughters of the grandfather will get two-thirds, as Allaah Says (what means): {If (there are) only daughters, two or more, their share is two thirds of the inheritance.}[Quran 4:11]. The remainder of inheritance will be equally divided among his six nephews [sons of brothers], as the Prophet said: "Give the prescribed share of inheritance to those who are liable for it and the remainder [of it] is for the nearest male blood relative." [Al-Bukhari and Muslim] Then, the share of his nephew [son of his brother] who died after him will be divided among his sons and daughters as we already clarified, the male twice the portion of the female.
Therefore, the inheritance will be divided to 162 shares. Each daughter of your grandfather will get 36 shares, and each nephew [son of brother] will get 9 shares. As regards the sons of the brother who died after the grandfather [his uncle], then each of them will get 2 shares and each girl will get one share.
Finally, it should be noted that inheritance is a very complex an issue, so one should not rely on a mere Fatwa that is issued according to the question, rather the case should be taken to an Islamic court, and if not possible then you should present it orally to the scholars so that they would investigate all its details. It might be that there is an heir whom you did not know that he is as such and so you did not mention him. It could also be that you did not mention some circumstances, for instance you did not mention to us in the question whether the sons of the brother are all full brothers or sons from father's side only, or that some are full brothers and some are sons from father's side only, because this affects the answer. Our answer is based on the fact that all of them are full brothers or brothers from father's side only.
Allaah Knows best.