There is currently no Linux/m68k port for several 680x0 based computers that would be able to run Linux. The reason for this is rather simple: Nobody has written it. The reasons for that are many: The people who already have most/all of the knowledge on the Linux side of the port are usually busy maintaining/improving one of the existing ports. Another quite common reason is that no or only insufficient documentation on the hardware of that platform exists. Sun 3s are an even more special case: Unlike all other machines mentioned here, they don't use Motorola MMUs (except the Sun 3/80, which uses a 68030).
See Chapter 5 for details on progress (or lack thereof) toward completing ports to these systems.
As far as I know, you can run NetBSD or OpenBSD on some of the unsupported systems.