hi,
i have a triple boot system with windows 10, ubuntu 20.04, and fedora 36. Yesterday i reorder fedora grub menu using grub-customizer after restart, grub menu only contain windows and ubuntu.
i have fedora boot efi installed in different partition and Ubuntu grub can’t find fedora partition.
now i cant access fedora since it not showing in ubuntu/fedora grub menu.
how do i boot to fedora again ?
If you can boot into Ubuntu, do the following:
1- Edit /etc/default/grub and enable OS-PROBER
2- Run sudo update-grub
3- Now your Ubuntu GRUB should have Fedora listed too.
4- Boot into Fedora from Ubuntu GRUB and run: sudo grub2-mkconfig -o /boot/grub2/grub.cfg
it should fix your Fedora grub.
I should note that I never ran a dual boot of Ubuntu and Fedora so I’m not sure if this will work.
Also, I suggest you to consider using distrobox on Fedora for your Ubuntu specific stuff, it allows you to run Ubuntu binaries on Fedora (or any other supported distro) and can even run GUI apps. I don’t know your use case but it may be worth if triple booting is inconvenient.
In my experience Fedora and Ubuntu, speaking of grub and os prober, don’t live together in armony… I wrote about a slightly different approach without os prober Multibooting with GRUB | Lorenzo Bettini
I have been using something similar as what you describe on a non-UEFI system to boot two different versions of Fedora. The same configuration is now working on a new UEFI machine with no change in configuration.
I would guess, though, that you need to use the Fedora grub as the first boot loader, and let it interpret the Ubunto, etc boot configuration files. I doubt that grub from the other distros can understand the BLS configuration as used with Fedora.
Also, in my opinion it is much easier to run the other distros in a virtual machine
It seems that many problems of this type crop up when using multiple efi partitions. As much as possible one should use a single efi partition so as to avoid these issues.
I agree about running other distros in a VM.
If you use Fedora as the first boot loader things are easier. However, if you disable BLS in Fedora you can do also the other way round (but then you have to remember to update Fedora grub configuration when you install a new kernel).
I personally run 4 distros in each of my two laptops and everything works great once you know how to configure grub 
In the approach I describe I’m using a single efi partition, mounting it as /boot/efi
Your description for this is great. However, the OP said this (which indicated having 2 different efi partitions.