“Therefore, did they teach in error, or did they teach what they were taught by the Apostles?”
This teaching of a triune god didn’t come from the Apostles so it was taught in error.
Which is not surprising, this Jesus foretold. (Matt. 13:26)
If such a group can be so easily deceived, and many have believed it since then, how can you be so sure you are right? Perhaps it is you who are being deceived.
In the whole of things, I will take the early church witness and their ability to discern the Word properly over your ability to discern the Word properly.
In order to not only say that but believe it is a true statement, one would have to totally ignore ALL the Scripture passages that have been posted in this thread so far. The teaching of the triune God was a doctrine ALWAYS believed and held to by the earliest Christians until this very day. The false doctrines followed afterward that denied Jesus is Almighty God in the flesh - the Son of God, the second person of the Trinity - and/or that the Holy Spirit is not the third person of the triune God. If not for the Scriptures - written by and distributed under the authority of the Apostles - then it would NOT have been a universally-held doctrine of Christians. Besides, it's not just a few nebulous verses we are talking about here but numerous references throughout both the Old and New Testament books.
Now, people can choose to ignore those Scriptures or quibble over their clear interpretation, but no one can honestly state the doctrine was not found in Scripture WRITTEN UNDER THE INSPIRATION OF THE HOLY SPIRIT.