One of my earliest awakening moments was finding out that the "Trinity" brochure was so full of misquotes and outright deception that it was profoundly embarrassing. I'm no theologian , but I would imagine that the Catholics would argue that the Trinity doctrine is biblical but is also backed up by later church writings and tradition.
I do remember ( reluctantly and foolishly ) agreeing to accompany a pioneer on a call she had with a Catholic theology student and we discussed the Trinity , amongst other subjects. The guy knew his bible far better than we did , and basically tore us apart.