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.
If the deceased did not leave any heirs except his father, his son and his daughter, then his father gets one-sixth of the inheritance as his legal share due to the existence of children of the deceased. Allaah says (what means): {And for one’s parents, to each one of them is a sixth of his estate if he left children.}[Quran 4:11]
The remainder will be divided amongst the son and the daughter by Ta’seeb (by virtue of having a paternal relation with the deceased and not having an allotted share, so they get what is left after the allotted shares have been distributed); the male twice the share of the female as Allaah says (what means): {Allaah instructs you concerning your children [i.e. their portions of inheritance]: for the male, what is equal to the share of two females.}[Quran 4:11]
Therefore, the inheritance should be divided into 18 shares, the father gets one-sixth, which is 3 shares, the son gets 10 shares and the daughter gets 5 shares.
Allaah Knows best.