God's oneness is not like anything on this earth; so, how can anyone make light of it to say they understand it as you imply with your question?.
Saying and believing God is one is not understanding the essence of God or to know what makes Him tick from our finite perspectives. We say and know He is one too. There are not three gods; there is only one. It is not my problem you don't believe what I know. God's tri unity is not like anything in His creation so we have no reason to reject it especially using the excuse that our salvation should depend on figuring God out God's perfect oneness as a 2 year old can do according to you.
We also say God is one with no partners or associates, and we truly believe this. How can non Christians tell us what we believe, when the Quran shows misconceptions about Christian beliefs as I have demonstrated over and over again? God is Spirit and He is Holy and His word is Jesus. They are perfectly one in a way that passes human understanding. We don't have to mention any messenger with the one true God to be Christian. Do you have to mention a messenger with Allah to be Muslim?