I think 28/50/90 is a good recommendation, and what I started with, but I rarely use my 90, because I do like to get in close. Still, it's good to have.
In general, a wide angle makes the picture look more like you are involved with the subject, surrounded by the picture and the subject, and really present in the situation. Wides tend to show background, both because their depth of focus is greater and because they take in a proportionally wider sweep of the background that can't be ignored in the picture, and so it needs to be somehow used as part of the picture. They exaggerate space and make a more exciting, but also more complex, picture:
Bootjack Equestrians by Michael Darnton, on Flickr
Art Institute by Michael Darnton, on Flickr
Longer lenses feel like you are standing at a distance, uninvolved and watching. They can also be used isolate the subject by throwing the surroundings out of focus. If you're concerned about the environment of a picture, use a wide; for isolating details, a longer lens. They're often used for portraits, to isolate the face:
Russell by Michael Darnton, on Flickr
The normal lens, the 50, stands in the middle, neutral to some extent, though it tends more towards isolating than encompassing. Think of it as the way you'd normally see, with a bit of surroundings, but still concentration on the central subject.
Liz, at home by Michael Darnton, on Flickr