I've been a long migraine suferer and neither of the three ever helped. They did not help at all with other minor health issues I had. About two years ago I switched to Krill Oil and the results were miraculous; the migraines almost disappeared and I have amazing energy!
This winter I also switched my toddler from cod liver oil (been taking this since 5.5 months, sometimes alternating with fish oil) and also noticed great results; he did not have once sick day, only a day with very very mild cold like symptons. And we did travel a lot overseas during the Christmas season where the swine flu was at its peak here in Europe (must have spent about 15 hours total in a plane).
I use the one by Dr. Mercola; before I got pregnant I was using the Krill for women (with EPO) and now the plain Krill. For my DS I get the one for kids.
Fish Oil-- made from the bodies of small fish, mostly taken as a source of Omega 3 fatty acids. Cod Liver Oil-- made from just the livers of the cod fish, and taken primarily for Vitamins A and D. Unfortunately, ALL non-fermented cod liver oil currently has added (usually synthetic) vitamins in order to standardize the dosage. Fermented Cod Liver Oil-- made from just the livers of the cod fish, but fermented in a traditional method. Green Pasture's website has a really good description of the process. Has no added vitamins. Taken primarily for Vitamins A and D and some not-well-understood "cofactors" that result from the fermentation process and likely aid in absorption. Sold by a few different vendors, but always includes the name "Blue Ice."
I realize that I've grossly over-simplified the issue here, but this is basically how I think of these in my head and thought I'd try to capture the basics. For WAY more info, search for "cod liver oil" on the Weston A. Price Foundation's website, or search this forum for the zillions of posts on the topic! Previous folks have been WAY more in-depth on the topic than I have.
At any rate, hope that helps clear it up a little.