The Hebrew word “echad”. Picture source: egrc.
‘(…) We need to examine carefully the Hebrew word echad.
At Genesis 1:5 we find in the first (echad) day, a single (echad) day, two aspects to this one (echad) day: evening and morning. At Genesis 2:24 we find that Adam and Eve become one (echad) flesh. At Genesis 3:22 the man and woman “have become like one [echad] of us,” confesses God. Dr. Robert Morey comments: “But they did not lose their personhood when they became ‘one’ with God.”1 At Genesis 11:6 all of the people are one (echad), united, yet remain many. At Genesis 34:16, 22, the Shechemites desire to become one (echad) people with the Jews. At 2 Chronicles 30:12 the LORD grants the people one (echad) heart in a compound sense (cf. Jer. 32:39). At Ezra 2:64 the Israelites (numbering 42,360) are described as one (echad). Therefore, for the author of The Trinity Delusion — or any other Unitarian-oriented individual — to claim that the Shema indicates that God is one (echad), i.e., singular, Person is teaching error, and in this case teaching damnable heresy.
The conclusion should be obvious. Dr. Robert Morey writes: “The passages above are just a small sampling of the many times אֶחָֽד [echad] is used of compound oneness. But it is enough to demonstrate beyond all doubt that the Old Testament, from the Law to the Prophets, used אֶחָֽד [echad] to express a unified or compound oneness,”  and not to a singular unit. Had the Hebrews intended to convey that God is one in Person, in a strictly singular sense, then they would have used the Hebrew word יתיר, a strictly singular unit, not אֶחָֽד, a compound unity. Dr. Morey asks, “Who would use אֶחָֽד?”
A Unitarian would never apply the Hebrew word אֶחָֽד [echad] to God because it means a compound or unified oneness. If the authors of the Bible were Unitarians, we would not expect to find אֶחָֽד applied to God. On the other hand, if the writers of Scripture believed that God was multi-personal, then we would expect to find that they would apply אֶחָֽד to God because this would mean that God is "one" in a composite or compound sense. As a matter of fact, אֶחָֽד is the only available Hebrew word they could use to express this idea. When we open the Bible, what do we find? We find that אֶחָֽד is applied to God. He is "one" in the sense of compound oneness. This is so central to the Old Testament concept of God that it is found in Israel's Great Confession [at Deut. 6:4]. 
source: Will Birch, “How Jesus Disproves Trinitarian Theology… and Other Such Nonsense” (williambirch).
 Robert Morey, The Trinity: Evidence and Issues (Iowa Falls: World Bible Publishers, Inc., 1996), 89.