Quick Anime Question

Random question, but there is so much more to come when I decide to stop being lazy.

Regarding the anime "A Certain Magical Index"/"A Certain Scientific Railgun"/"A Certain Scientific Accelerator"

Are all of these tied together?
By that I mean do I need to watch "Magical Index" in order to know pertinent information about "Scientific Railgun" and "Scientific Accelerator?"

Or are they all independent of each-other?

If the latter is the answer, which series is recommended?