There are no authentic/uncontested Bible verses that prove a Trinity. The Trinity doctrine is arrived at by inferences - not unlike the way that JWs arrive at the doctrine that Jesus is Michael the archangel. The JW proof texts against the Trinity are at least as strong, if not stronger than, Trinity proof texts. When I had the JW mindset, the Trinity was really the least difficult teaching to refute. No scripture or argument that any Trinity believer ever said to me ever came close to getting me to doubt. So I think arguing the Trinity with JWs is actually quite enjoyable and much relished by them. It bolsters their faith that they have The Truth while the Trinitarians of christendom are so lost in believing man-made pagan teachings. So don't go there, don't do that.
Proving the Trinity is irrelevant. What is very relevant and what I think will have a much bigger impact on JWs is demonstrating to them, using their own Bible, the attitude that first century Christians and NT writers had toward Jesus. Show them the scriptures where the NT praises and honors Jesus to a degree that the JWs find very uncomfortable and disconcerting. Scriptures like 2 Peter 3:18 and Revelation 5:11-14. Point out the contrast with their attitude toward Jesus. It's not about whether or not Jesus is God or part of a Trinity - make this point clear to them. It's about whether or not they're honoring Jesus sufficiently, in harmony with the example of the NT writers and first century Christians. Ask them, rhetorically, what other JWs in the congregation would think of them if they started honoring Jesus in the manner that NT writers did. Get them to see that there is something very wrong about their view of honoring Jesus and ask them who would want them to not honor Jesus in line with the honor that the scriptures show him receiving.
Another approach can be to ask them why it is that there is not a single verse in all the NT - especially the epistles - that raises the issue of Christians wrongfully worshipping Jesus and being counseled against it. Mention the fact that this is very strange given the context that first century gentile Christians were coming from cultures steeped in polytheism where the worship of gods and their sons were relatively common, thus it is inconceivable that there would not have been gentile converts projecting this former way of worship unto their newfound faith of Christianity with many starting to worship Jesus. It is inconceivable that the influx of gentile Christians would not have seen this problem arising often enough or seriously enough as to warrant even a singular mention in all the NT. Yet, there are verses in the NT that are ambiguous enough to be interpreted or misinterpreted as legitimizing the worship of Jesus. So if the worship of Jesus is wrong and since the worship of gods and their sons was rampant in the first century how could the NT be so silent on this issue of the wrongful worship of Jesus even while containing verses that can so easily be seen as legitimizing his worship?
I think these approaches would be better than trying to argue a Trinity with them.