Praise be to Allaah.
The mortal remains of the martyrs are like those of other people, the mortal remains of each martyr should be buried in a grave of his own, unless it is too difficult to bury each one on his own, because there are too many dead owing to disease, killing etc. (In that case) it is OK to bury two or three in one grave, and the one who was best in religious commitment should be placed closest to the qiblah, as the Prophet (peace and blessings of Allaah be upon him) did with those who were killed at Uhud.
And Allaah is the Source of strength. May Allaah bless our Prophet Muhammad and his family and companions, and grant them peace.