The TRINITY is a difficult subject to explain and a difficult “entity” to understand, even the most educated of theologians (Father Corapi) openly admits that full understanding is impossible in our human condition, and it will not be until we leave our bodies and enter into our Judgement and salvation that the TRINITY will understood.
And the jw will surely point out that the word “TRINITY” is not found in the Bible, which is true, but in place of the word you find the actual TRINITY appearing at JESUS baptism with the JESUS, the HOLY SPIRIT in the form of a dove, and GOD the FATHER speaking from heaven, “This is MY beloved SON in WHOM I am well pleased”.
So, in effect the jw is asking you to show him something that you cannot do, or explain something that does not have an earthly example - the closest would be water in three forms of ice, vapor and liquid, but it is still water.
However, the jw cannot “show” you gravity, because it is an invisible force. He can show you the effects of gravity, as you can show him the effects of the TRINITY as indicated above.
At the end of the day, howver, it will actually be The HOLY SPIRIT that moves the jw into the understanding (faith) of the TRINITY, so pray for that intercession