Exercise 1: Separate Root Partitions, One Shared EFI System Partition — Possible Solution
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Explanation:

Each operating system's ROOT filesystem holds that OS's own complete,
independent set of files -- its own kernel, its own installed
packages, its own configuration -- and these are fundamentally
incompatible with each other at the filesystem level (Debian's root
uses ext4 with a Debian-specific directory layout, Fedora's uses its
own separate ext4 instance with its own layout, Windows uses NTFS
entirely). There is no reasonable way for two operating systems to
share one root filesystem without corrupting or overwriting each
other's own files, so each one genuinely needs its own dedicated root
partition -- exactly matching grub1-6's own point that Windows and
Linux "each need their own partitions."

The EFI System Partition is different in kind, not just in name.
Per grub1-6's own material, it holds only BOOTLOADER files -- small,
independent .efi executables, one per operating system, each simply
sitting in the same FAT filesystem as separate files in separate
subdirectories (e.g. /EFI/Microsoft/ for Windows, /EFI/debian/ or
/EFI/fedora/ for the Linux distros). Since these are just individual
files coexisting on one ordinary filesystem -- not competing sets of
an entire operating system's own files -- there's no conflict in
having all three bootloaders' own files stored together on one shared
partition, and grub1-6 already established this coexistence is
exactly how UEFI's own multi-boot support is designed to work.

WHY THIS WORKS AS AN ANSWER
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This explains the real reason root filesystems can't be shared
(incompatible OS-specific file layouts) while the EFI System Partition
CAN be shared (it holds small, independent bootloader files coexisting
on one ordinary filesystem, not competing operating systems), directly
tying the explanation to grub1-6's own established material.