If you understand the nature of god of being invisible and untouchable and who scholars believe jesus was in the old testament ( the word of the lord or the angel of the lord), it's really not hard explain.
The new testament explains who jesus was in quite simple terms. The visible image of the invisible God or no one has seen the father only the son. Not really how they get a third member but the concept isn't as mysterious as some would like to believe.
Now if God comes to earth as a human limited by his finite mortality is he really god? The arguments at that point just become idle circular philosophy that really goes nowhere. Watchtower reasoning.