One definition of the Trinity in Christian theology is that God is "one substance but three persons."
So, the Father is fully God and the Son is fully God and the Holy Spirit is fully God, but the Father is not the Son or the Holy Spirit, and the Son is not the Father or the Holy Spirit, and the Holy Spirit is not the Father or the Son.
Confusing? Indeed it is.
But here is an illustration that may help a little bit. Like all analogies for God, I'm sure it breaks down at some point, but I find it helpful in getting a glimpse of how each member of the Trinity can be separate but "one substance."
Consider this illustration:
It is a picture of a young woman AND it is a picture of an old woman. (If you don't see it, look at it for a while.)
Two different people.
But the picture is not half old woman and half young woman; the young woman is the whole picture and the old woman is the whole picture.
However, the substance (the pixels, or the ink on paper) is identical.
One substance but two distinct people.