How to properly set up multiboot Windows, Ubuntu and Kali Linux?

Question

Iron

Hello!
Installed Windows 10, then Ubuntu 16.04, and finally Kali Linux. GRUB now has a look specific to Kali Linux. All 3 systems booted fine, but Ubuntu had its own GRUB in the /boot partition.
I decided to tweak the menu items a little, assembled GRUB Customiser and edited everything from under Kali. After saving, WIndows stops loading every time I do something in Ubuntu/Kali. And GRUB seems to work just fine, because. Windows is loading, and then the computer just reboots and so on ad infinitum. Fixed via the Windows Installer console (bcdboot E:\Windows, bootrec.exe /FixBoot).
There is only one physical disk, the output of fdisk -l is: pastebin.com/u38Gnejz
GRUB is installed on /dev/sda, the computer does not support UEFI.
Tell me how to fix the problem? Thanks in advance!

Answer 1

What for? Is the board so old that it can't boot from USB?