What qualifies the 3 entities of the Trinity as one? Follow up to this question: (If A = x and B = x and C = x then why isn't A = B = C? (Trinity)

What qualifies the 3 entities of the Trinity as one?

Now that the concept of the Trinity is more comprehensible to me - that the Trinity is not one entity with three different aspects, but rather three entities with different personalities with a shared essence and relationship with each other (perichoresis)

There's a very minor yet SIGNIFICANT semantic nuance. My friends and I are human (human as in adjective, denoting our attributes), but we are not ONE human (human as in noun, an entity that possesses the quality of being human). My friends and I share the attribute of being human, but we are not one human.

If I for instance take three laptops of the same model, I could say that they are the same laptop, in the sense that they are the same model - created by the same manufacturer, share the same components and OS. However, we are still talking about three laptops, not one.

As such, Christians call the Son, the Father, the Holy Ghost God (God as in adjective, denoting the attributes of divinity to each respectively), but the Son, the Father, and the Holy Ghost wouldn't be ONE God, would they?
(Of course I understand Christians cannot proclaim to believe in three Gods, but is this logically consistent?)

Another analogy: Say I abolished the power hierarchy between all the Greek deities and gave them the same essence, but kept their specified roles and relationship with each other. I claim that they are one, but for all practical purposes they really are not "one". That's how I understand the Trinity

Then by which standard do we classify them as one? What is it that makes them out to be one? in which sense are they one? Is it the close-knit relationship they exhibit with each other? 

This link shared in the comments conveys my understanding of the Trinity pretty well

If I may interject my personal opinion, I believe Christians are trying too hard to market this as monotheism. This may not be tritheism per se, but this is not monotheism either, the Trinity deserves its own theological category.

EDIT: The attached question is not the same as mine. The attached question merely seeks an explanation of the general concept of the Trinity, while I request people to directly address the question as to why/how the triune personalities of the Trinity constitute one God.
What qualifies the 3 entities of the Trinity as one?

There are not three entities or beings—three Gods. There are, within, or intrinsic to, the one God (or rather the one God is definitionally in some respect three; namely, He is tri-personal) three 'somethings.' They happen to be revealed to be quite Personal.

Put simply, 'trinity' describes something of the intrinsic nature of God; 'one' describes the numeric total of essences that are called God and have the attributes of God: there being but one, (Deut 6:4) who is known variously by titles YHVH, the God of Abraham, Isaac and Jacob, etc.

So they are one because they are intrinsic to each other and co-definitional (like how, analogically, 'Father' is meaningless without 'Son')—each no less than the other being definitional to the one essence of God: God is eternally three in some sense, yet in that He is but one, only God, He is one and only.

The name or title 'God' can, then, understandably, and readily, be attributed to any of the Persons of the Holy Trinity because each is no less God than the other, since they are God for the same ultimate reason: their essence, nature, being is one and the same: the divine and eternal, ineffable Deity, the only true God.

Scripture happens to use a convention whereby the Father, since He is the ontological beginning of the Godhead (not the temporal—the Son and the Spirit are not created, especially since they are eternal, but rather 'Son' simply 'logically' or 'onotologically' follows 'Father' and so forth: the economy of the Godhead is such that the Son and the Spirit recieve being God from the Father, not that they are created) is used to simply refer to the one God. But since the Son and the Spirit are directly of the same substance of the Father, they are just as much God as He: there was no time when the Father was God without the Son and the Spirit being God.

There is 'one' God because 'one' describes not the number of Persons God has, naturally, but the number of Gods. There is but one God, who is, eternally, tri-personal. And ineffable, so that all excercises in forcing God to conform to the current human understanding of the world etc. is quite futile. In fact, we only know God by analogy alone.