Triple Boot Ubuntu, Kali Linux & Windows 10 Grub issue
Solution 1:
I would recommend you to run a live cd of Ubuntu and install boot-repair. Open the terminal and copy and paste the following command:
sudo add-apt-repository -y ppa:yannubuntu/boot-repair && sudo apt-get update
After successfully adding and updating the repository type the following command:
sudo apt-get install -y boot-repair && boot-repair
There you can solve your issues related to boot.
Or try it with Super GRUB 2 disk.
You can download it from here.
Then install it in a usb using Unetbootin Now you could restart your system and make sure it is set in the boot option menu.
Solution 2:
If you want to use boot-repair you dont actually have to install it. You can boot from the boot-repair live CD. I have had good results in the past using the live CD, simply selecting the automatic repair option.
However, why not try this. Boot into your Ubuntu and re-installgrub from a command line.
Assuming you have one HDD /dev/sda
sudo grub-install /dev/sda
You might also have to update grub.
sudo update-grub
That should install Ubuntus grub to the MBR.
Solution 3:
Others have suggested using Boot Repair, and this may well work. If it doesn't, you could try my rEFInd boot manager, which eliminates several (mostly minor) GRUB-related problems when dealing with multiple distributions. rEFInd is also more reliable than GRUB at booting Windows. You can try rEFInd using a USB flash drive or CD-R (versions of both are available from its downloads page) risk-free, and if that works, install the rEFInd Debian package or PPA in Ubuntu.