The problem with belief in the Blessed Trinity is that to a Muslim it looks like nothing but another form of shirk - that is, it denies the Unity & Uniqueness of God by associating partners with Him, who are by definition not God. Therefore, what Christians believe is just another form of polytheism - therefore, it has to be rejected.
If we do not mean by “circle” what “God means”, He might by “good” mean what we mean by “evil” - if a revelation is to be helpful to its recipients, there has to be some likeness between God’s “idea” of good & ours: otherwise His revelation will be misleading. This is why emphasis on the genuinely human authorship of the Bible is so important to understanding it: what God means, is not accessible to human testing - at least in principle, what S.Paul (say) meant by his choice of words can be tested: versions of his writings can be compared, analysed, tested, criticised. Even though his personal knowledge of Christ was his & not ours.
Since we have access to Christ through the Holy Spirit of Christ, we can (as the Psalmist says) “taste and see that the Lord is good” for ourselves, both individually & as a Church. IOW, we can test the truth of St. Paul’s word for ourselves. That so many Christians through the centuries have found them verified in their own experience, is a strong evidence of their truth - & so, of the truth & authenticity & Divine Authorship of them

A single individual might be wrong, & often is - but when many individuals taste and see for themselves that what seems incapable of being true, is found true by every test that life can put it through, the case for the truth of whatever-it-is, looks stronger: which is how the fact of a Church is so helpful - it is immensely varied, in just about every way conceivable: the one bond common to all its members is that they are Christians. That they believe in a God Who is Tri-Une, despite their many differences & enormous variety, is something worth noting.
And as his words can by these tests be known to be true, we have a “test” for the Church’s experience that the Blessed Trinity is a reality. Faith is always necessary while we are on earth - perhaps because Christian faith is faith by persons, in Persons. So it is simply not possible to demonstrate that God is Tri-Une by argument: relations between personal beings cannot be tested in that way; that the Blessed Trinity is “really Real”, is not a theorem in geometry; so an argument that would suit geometry is an inappropriate method for knowing the Blessed Trinity.
An answer based on the freedom of God,like “X defies human logic” is an answer vulnerable to attack because it is not clear to an anti-Trinitarian that the Christian belief is itself anything more than a construction of human reason. We believe in the Blessed Trinity because of God’s Revelation on Christ, Who is made known by the Divine gift of faith - but, that reply is not going to answer a Muslim: as belief in the Trinity is not true (the Koran makes that clear enough), nothing can make it true. The absolute trustworthiness & Divine origin of the Koran is basic to Islam; it can no more be taken out of Islam without destroying it, than belief that there is a God can be taken out (say) Christianity without destroying that.
In the Doctrine of God, the Christian certainty, & the Muslim certainty, have somehow to be interpreted as compatible with one another. (That, or one of them must give way: which is not likely to happen.)