Loosing inteet connection when I add DT computer win 10

I loose my inteet connection to, 2 win 7 computers every time I add my latest (used) computer with win 10,I have tued off all wifi on them. . All 3 Desktops are Not on a network and my Century link Modem has 4 Etheet ports.I can run it by itself and no problem. I can run the other 2 at the same time and no problem.. But if I add the 3rd.. It slows down to a crawl and then knocks me off the Inteet.

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