Would depend entirely on your definition of worthwhile. 3 toons is good for PVP (both arenas and BGs), you can run most of the PVE content and I would assume even some of the harder raid difficulties are viable to 3-box with a competent raid group. Finding one to take all of your toons along is most likely an issue though.

You're not very limited in what you can and can not do really. Some things will be much harder than others naturally, but it's pretty much all out there for you.