Proposition # 3: Though Deuteronomy 6:4 says that God is one, the Hebrew term translated one indicates a compound unity. Therefore, the one God must be a multiple in some sense, that being revealed by other passages of Scripture as a Trinity.
Response: The language in which Deuteronomy 6:4 was originally written was the native tongue of the Jews, yet they have never derived the concept of a triune God because of this verse, or any other passage of Scripture. Notice what Jesus said to the Samaritan woman, “You [Samaritans] worship what you do not know; we [Jews] worship what we know, for salvation is from the Jews.” If God were triune when the Jews held no such concept, wouldn’t that have made Jesus’ statement to the Samaritan woman inaccurate? — John 4:22
The assertion that the Hebrew word for one implies a compound unity is simply untrue. Any implication of plurality is drawn from the context. For example, where Genesis 2:24 says, “They will become one flesh” the plurality is drawn from the pronoun “they.” Notice that Genesis 1:9 reads, “Let the waters under the heaven be gathered together unto one place.” How many places? One. Numbers 6:14 reads, “And he shall offer his offering unto the LORD, one male lamb.” How many lambs? One. Was this lamb a trinity or a compound unity?