they are:
Judaism and Christianity.
Abraham
That might refer to Abraham.
JudaismChristianityIslam
Abraham fathered these religions: Judaism, Islamic and Christianity
Abraham is considered the person central to the holy covenant in all three major religions - Judaism, Christianity, and Islam. God promised Abraham descendants and land, and this covenant is considered foundational to the beliefs of all three faiths.
Abraham, the father of three religions was a tent dweller. See related links for information on Abraham
Abraham and Moses are considered prophets in the Abrahamic religions of Judaism, Christianity, and Islam. They are revered figures in all three faiths for their leadership, teachings, and guidance in spreading monotheism.
If "we" is "Jews and Christians", the answer would be Isaac. If "we" is "Muslims", the answer would be Ishmael.
The common ancestor of the three religions Judaism, Christianity and Islam was Abraham
Judaism, Islam, and Christianity.
We are all linked as Abraham is the father of all three religions.