The word "echad", depending on it's context can either refer to plural oneness or singular oneness. For example Adam and Eve become echad in Genesis 2:24 or one and the Tabernacle parts become echad in Exodus 26:6.
The Shema contains a singular usage, referring to how many Gods exist, namely only One God to worship, YHWH and no other.
Before the Unitarian can clap their hands and think they have won the debate if they read this article, read the following articles:
Regardless of the Shema using Echad as singular or Yachid, this would not disprove nor prove the Trinity, as that is not the goal of the Shema. It's a point missed by Trintarian and Anti-Trinitarian alike.
There are other cases of Echad being singular, such as Isaiah 51:2.
For evidence that Echad CAN have a singular and plural meaning from a counter missionary, Tovia Singer in his lecture "Is The Trinity Biblical?" talks about this: https://www.youtube.com/watch?v=_Joz6vKSh1o, Watch from 15:56-23:40. Singer later claims that the arguments used by Christians are used by Pagans also but to be very honest, Pagan Polytheism and Biblical Monotheistic Trinitarianism are NOT the same thing and any claims they are the same thing are a strawman of Trinitarian belief.
However, the goal of this article is not to prove the Trinity and this is already done elsewhere on my blog.