Kharijah bin Zayd (may Allah have mercy on him) narrates from his father (may Allah be pleased with him): If the deceased leaves three grandmothers of the same degree, and there is no mother or father besides them, the sixth will be for all three of them; and they are the maternal great-grandmother and the paternal great-grandmother.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12353