Gen 1 , 26
Then God said: Let us make man in our image, after our likeness.
Isaiah 6, 3
Holy, Holy, Holy is the Lord of Hosts!.
Isaiah 48, 16
Come near to me and hear this!
Not from the beginning did I speak it in secret;
" Now the Lord, God has sent me,
and his spirit.
Matthew 28, 19
Go ,therefore, and make disciples
of all nations, baptizing them in the name
of the Father,and of the Son, and of the
holy Spirit, teaching them to observe all
that I have commanded you. And behold,
I am with you always until the end of the age.
1 John 5, 7
So, there are three that testify,
I have posted these scriptures elsewhere to indicate a case can be made for the Trinity as being Biblical. More examples could probably be found but may not be necessary. I think the biggest problem arises when people can't quite understand how it can be and therefore it must be false rather than accepting the possibility of their own lack of understanding. We christians take much by faith but if we didn't or could not, we would have to rely on our own logic to accept anything which is exactly how many approach the question of the Holy Trinity. We can't answer some pretty basic questions about the ' raison d'etre' of most of existence except by faith and now we are supposed to be able to understand the complete nature of the architect of existence? I think not. ( please do not paraphrase me in that last sentence haha).
I believe God has put so much before us in the world and universe to discover as a gift to us. But nothing greater than Himself and if we can admit we don't know everything about creation we should be able to admit we don't know everything about Creator.