If there is only one God how do you impute to Him a “triune nature”? Three is not one and one is not three. The oneness of God has no parts and is not divisible.
You are right that one does not equal three and three does not equal one. However, that’s not what the doctrine of the Trinity states. (even though it may look like that at first glance.) We believe there is one, eternal, infinite, Being - God - who exists eternally as three distinct persons.
Now, that’s a contradiction only if we don’t understand what is meant by the words being, nature, and person. But once we do that, it becomes a little clearer. Basically, nature refers to “what” something, while person refers to “who”. A being is anything that exists. So, for example, if you asked me what I am, I would say I am human, because I have a human nature. OTOH, if you asked me
who I am, I would say Robyn. But though there is obviously a distinction, I still am only one being. I’m not two beings. I’m also not divided.
Same thing when it comes to God. We believe the Divine Nature is absolutely one, indivisible, simple.
There is only one God. But just like distinguishing between my nature and person didn’t introduce any division inside of me, so it is with God. There is only one divine nature - one divine being -
and there are three persons who each posses that
same undivided divine nature perfectly and completely.
Now, the hard part comes when we try to comprehend how the three persons can all possess the same divine nature without dividing it. A lot of theologians have pondered that, but ultimately, (and I know this might sound like a cop-out here, but it’s true) it is a mystery. It’s above and beyond every experience we have ever had. BUT, that doesn’t mean it’s therefore contrary to to logic and reason, and here’s why:
The ratio between nature and person for human beings is 1:1, right? Since that’s all we have ever known, it’s natural to conclude that’s the only way it can be. But is it? Nope, because even in nature, we see it’s possible for a single being to have a different ratio and still be one being. A dog is a being, it has a canine nature, but it’s not a person. So now we have a being with a ratio of 1:0. Given that, I don’t see any logical reason why God can’t have a ratio of 1:3. Why couldn’t he be the One Divine Being who exists as Three Divine Persons from all eternity?
Yes it’s true, we can’t comprehend it, and yes we have no experience of anything like that. But that’s exactly the point. God in his very essence
should be so far beyond our experience and comprehension. He’s the infinite and Almighty one after all.