If we say God is personal, He is ONE PERSON, not three. But in that ONE PERSON - there are three subjects about which statements can be made. Each subject is personal, because each is God and God is personal.
The best analogy I can think of is to say that I can speak of -- My Memory, My Understanding, My Will. Each of these is a subject about which statements can be made. And each of these is personal, not because I am three persons but because the single person I am energises in these three ways. My memory is me, my understanding is me and my will is me. The Father is God, the Son is God and the Holy Spirit is God.
OK, but why did people see this belief as the very secret of life? Why were people prepared to die for this doctrine? The doctrine of the Trinity was felt to meet two threats, each of which, if fully implemented would destroy us. It was something like a promise that these two threats would finally be overcome.The two threats are --- ISOLATION AND ABSORPTION. Each is a potential murderer.
ISOLATION: Relationships with people offer good. They are our response to the threat of being isolated. A great deal of what we do is motivated by the need to take precaution against ISOLATION. Sex, Ambition etc. etc. etc. Death is feared as the final isolator, the dread of aloneness.
At the same time there is the threat of being ABSORBED into what promises to extinguish our individuality - the demands for continuous attention - people who demand and monopolise conversation to prove they haven't suddenly shrunk into the furniture - people who are constantly seeking praise - people who think that without them everything will collapse. Death again is feared as the final absorber.
ISOLATION, ABSORPTION - each capable of exterminating us but then the ultimate threat: EXTERMINATION, the triumph of non-being over being.
In the doctrine of the Trinity we have set before us an image in which this threat of non-being, in its two chief forms is met and overcome.
ISOLATION is overcome because the one God is described as being eternally in relation. God is Father, Son and Holy Spirit. Our hearts protest that we were not made for death which comes from being isolated. What better picture than the God who, although He is ONE is also eternally THREE.
At the same time the doctrine provides us with an image in which the opposite threat is met and vanquished. The persons of the Godhead are forever distinct and unconfused. In being itself this threat of non-being is forever encountered and forever conquered.
This is what made Trinitarian doctrine living faith and ultimate concern. If people feel after God and find Him, they do so at a level of their being far deeper than reason. For the heart is wiser than the head and knows more.