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.
The mother is entitled to one-third of the estate after the husband receives his share. Then, the father gets the rest of the estate by Ta‘seeb (i.e. by virtue of having a paternal relation with the deceased and not having a specific allotted share), according to the opinion of the majority of scholars, including the four Imaams. This case is known as one of ‘the two ‘Umariyyah cases', named after ‘Umar ibn Al-Khattaab .
Thus, the estate should be divided into six shares; the husband gets half of them, i.e. three shares, the mother gets one-third (of the rest of the estate), i.e. one share, and the father gets the remaining two shares.
Allaah knows best.