That's my question. When, exactly, do the 3 kings fall?
At the rise of the anti-christ
Daniel 7:8
"While I was contemplating the horns, behold, [Dan 8:9] another horn, a little one, came up among them, and three of the first horns were pulled out by the roots before it; and behold, this horn possessed eyes like the eyes of a man and [Rev 13:5, 6] a mouth uttering great boasts.
(Whole Chapter: Daniel 7 In context: Daniel 7:7-9)
Daniel 7:20
and the meaning of the ten horns that were on its head and the other horn which came up, and before which three of them fell, namely, that horn which had eyes and a mouth uttering great boasts and which was larger in appearance than its associates.
(Whole Chapter: Daniel 7 In context: Daniel 7:19-21)
Daniel 7:24
'As for the [Dan 7:7; Rev 17:12] ten horns, out of this kingdom ten kings will arise; and another will arise after them, and he will be different from the previous ones and will subdue three kings.
(Whole Chapter: Daniel 7 In context: Daniel 7:23-25)