Again I sort of have to remove the theological aspect because i’m not educated enough to speak on it, only the way you’re thinking here. You assume for two separate entities to be one they have to literally merge or be absorbed to form as one. Let me put this in a different example.
I have a green apple, an orange, and a pear. They are all separate entities and hold a lot of traits that make them such. Their color, their taste, etc. However are differentiation of them is due to our restrictions on how we define things, through forming conceptions based on our senses. However removed from those none of these three fruits are separate. They are fruits. (I could expand upon the example more but i’ll stop.) The separations we make are simply inventions of the mind. However, these dualities do not exist separate from each other but at the same time. We may choose to look at an apple from an orange, or look at both as just two fruits, but how we perceive them does not alter the state they exist in, which is both at the same time.
To try to apply this back onto the religious argument, it seems the distinctions we make between the father, son, and holy spirit all rely on our tendency to think in dualities and invent separations when, removed from our senses, they are the same. To try to expand on this, and commit complete heresy in the process, it’s like thinking the spirit of jesus christ didn’t exist before his birth, or that his spirit ceased to exist after his death. It is a constant and can coexist in multiple places at the same time.