The differences between these three civilizations is more striking than their similarities. All three were river civilizations, that is dependent on particular rivers for their survival; all three were monotheistic, but similarities end there.
The Egyptian Civilization revered the Nile; hymns to the Nile such as the following were commonplace:
Hail to thee, O Nile, that issues from the earth and comes to keep Egypt alive.
He that waters the meadow which Re created.
He that makes to drink the desert….
He who makes barley and brings wheat into being…
He who brings grass into being for the cattle.
He who makes every beloved tree to grow.
O Nile, verdant art thou, who makest man and cattle to live.
The Nile was important not only as a source of life; but also served to protect Egypt; its cataracts upstream served as a barrier to invasion. It's floods replenished the soil in ancient Egypt, thus ensuring the continuation of the civilization. The Tigris and Euphrates Rivers of Mesopotamia also flooded, perhaps giving rise to the Epic of Gilgamesh and the story of Noah and the Flood; however it provided no protection; in fact they were easily negotiated which made the civilizations of Mesopotamia susceptible to Invasion. The early Indus river civilization, the Harappan or Dravidian civilization, was dependent on the Indus River, but there is no evidence of its flooding as significant; nor did it serve as a source of protection from invasion.
Egypt perhaps had the most elaborate religious structure; based on Re, the Sun God. The Pharaoh himself, ruler of Egypt, was considered the human representation of the God Horus. Egyptian belief in life after death conceived of rebirth in another realm, based upon one's life on earth. Mesopotamian Religion was based on the worship of gods of agriculture; primarily Baal, the God of rain and the storm; and Astarte the goddess of fertility and reproduction. Their religion often involved fertility rites which became quite erotic. No such practice is noted in Egypt. Nothing is known of Harappan religion, as their language has not yet been deciphered; however the Aryans who followed them worshipped a pantheon of Gods, the most important of whom was Indra, the god of War.
A final difference between the three is their system of writing. Egyptians had several alphabets, the heiratic, demotic and finally heiroglyphs, or "sacred writing." All were typically written on a paper like substance made from reeds known as papyrus. The Mesopotamian system of writing was known as cuneiform, which was written on clay tablets. Early Indus civilization, notably the Aryan had no written language for some time, but used a sacred language known as Sanskrit, and a secular dialect known as Prakit. Much later, legends and myths were reduced to writing in a document known as the Vedas, committed to writing about 600 B.C.E.