Muslims would say Christ is not God and there is no such thing as a Triune God. This is mostly because they do not understand it. Well, just because you fully do not understand it, does not mean it is proof that it is not true. For example, take a look at a question which Christ asks:
Matthew 22:42-46>"What think ye of Christ? whose son is he?" They say unto him, The Son of David. He saith unto them, "How then doth David in spirit call him Lord, saying, 'The LORD said unto my Lord, Sit thou on my right hand, till I make thine enemies thy footstool?' If David then call him Lord, how is he his son?" And no man was able to answer him a word, neither durst any man from that day forth ask him any more questions. (Also in Mark 12, Luke 20).
Right here we see an example of something which completely boggles the mind, yet it is still true regardless of people's opinions. It is not a human's job to try and fully comprehend God. Therefore, Christ Himself refutes the logic of Muslims.