Explaining how three gods can be one..... it will end one of two ways.
1. A silly analogy.... "Water can be a liquid, gas or a solid and god can be god, man and spirit." The resson an analogy is reauired, not becaue the listener is intellectually challenged and requires help, but the point being made is so illogical, irrational and unproven an analogy is required to distract away from the original issue. Instead of talking about how a god can be three figures, the converstaion goes to how water can be in three states .... it's irrelevant and deceitful, it's amateur 'slight of hand' at it's most unimpressive.
2. State ... "As humans we don't have capacity to understand the trinity!" .... well STOP pretending you understand it then. STOP 'teaching' other people something that you don't and can't understand. STOP pretending you have any evidence, logic or rationale on your side.
Let's not even get into how you can be sure that the bronze age writings from ancient sheep herders and mystics are reliable, wheras all other contemporary writings or modern day science is not.
Have your faith, enjoy it, let it enrich your life.... maybe trying to preach to people largely attempting to leave a cult, is a little unsavoury however.