if we assume that the beatific vision, as the Latin Church says, is the same as seeing his face, then we commit a dangerous error.
It is a clear and OT and NT idea that the face of God (His essence) is veiled. God says to Moses that he can't show His face to him, Moses would have died - in other words, Moses would have lost his individual existence, being absorbed into God's essence. It is like touching the sun: you would be immediately melted by its 6000 °C temperature and lose your personal existence, being absorbed into the solar plasma. Yes, you would become "the sun", but you wouldn't be conscient at that time: you just wouldn't exist anymore to witness what's within the sun. That doesn't make the sun unknowable. Even if we can't see and touch the sun "in its essence", we can see it "in its energies", that is in the light and heat of the sun. The sun is literally "veiled" in light, in the same way as God is "veiled" in His glory. This shining light is what we can partake of the sun... so that the invisible sun is make intelligible in his light. Now, taking your question whether God is tri-personal, or we should count for the graces as a fourth person, I'll make a further astrophysical example.
Let's assume there's a system of three identical stars very very far from us. They are connected with each other by an intense gravitational field. They are identical in substance (they are all made of solar plasma) and form (they have the same mass and volume). These stars all shine in the same light: even if they are independent sources, light is always made by the same photons, which are identical in every part of the universe. Now, these stars are so distant that from the Earth that we see them as one star shining in a white light. Now the questions are:
Is this light a fourth star? The answer is: no, the stars remain three.
Are there three different substances in the stars? The answer is: no, the solar plasma they are made of is the same, so the plasma substance is equal.
Are there different lights from the stars? The answer is: yes and no. Yes: because every beam of white light conveys seven different colours, from red to indigo, at the same time; no, because the light produced independently from the three stars are made of photons, which are all identical.
Now take the comparison with God and answer these questions:
Is God's glory a fourth divine person? - No, the persons remain three.
Are there three different substances in the divine persons? - No, the stars are of the same substance/essence.
Are there different energies from the divine persons? - Yes, because the same glory is multi-faceted in different energies (love, grace, mercy, justice...), and no, because the energies are an emission of light from the inners persons, and since the persons are of the same essence and form, the energies irradiated by three persons are identical to each other.
Hope this helps... I know it's complicated, but I tried to examplify it as much as possible.
The fact that the sun and, more in general, light, have been so largely used by God in the Scriptures and in the mystic experience of the Church as symbols for Himself is a testimony of how studying the nature of stars can help us understand the nature of God. Our world orbits around the Sun like our lives should orbit around God... The sun provides light as a guide for our physical steps and heat to warm our bodies and sustain life, like God provides his energies to guide us in our spiritual steps, to warm our hearts with His love and to sustain our spiritual life. I love to call the sun "the mirror of God in nature" LOL
In Christ, Alex