Did you mean 17.04 ? Ubuntu site can't have link to 17.10. Only 17.04
PCH option under the PCI-Express speed works on PCI-E X16 slot 0. But I am using PCI-E X1 slot 2,3.
Does it change anything?
Yeah, 17.04LTS.
You need to set all the slots to Gen 1 or Gen 2, whichever one works. Otherwise, you'll get weird Nvidia display errors.
For example: Asus Prime H270 Plus - when it is not set to Gen 1 - on either Gen 2 or Gen 3 you get display errors, before the log in page.
And, only 2 cards are recognised - even though motherboard support is for 4 GPUs on PCI-E 1X.