dismiss would be apt here
Treat as unworthy of serious consideration (Oxford)
You can also brush aside/shrug aside the argument. (IMO, aside
is less strong than off
, which you can use.)
Arguments can be weak. I don't think weaken
is idiomatic here, but you can paraphrase it:
Point three is actually really interesting, but weak/thin/flimsy (or overstated), as shown by [counter arguement].
In case only a single word that means 'to reduce' will do, you could use undermine, (maybe)
Lessen the effectiveness, power, or ability of, especially gradually or insidiously:
I'm not sure it works here, though.
For a single word that means 'to deal with', you could
pursue/examine/analyze the argument. The ngram shows that pursue
is the most idiomatic one.