Kodak sells an "18% gray card," typically used to reflect "middle-gray" or "Zone V." In the film days, it was used as an exposure reference: I once had to shoot an 18% gray card on 35mm black and white negative, develop the film, then measure the emulsion's transmittance on a densitometer--an 18% gray card, exposed and developed properly, should read "0.75." Gray cards were also a helpful aid to those learning the "Zone System," a method of correlating both under- or over-exposure, and under- or over-development, for maximum highlight and shadow rendition (based on the existing scene's contrast ratio) in black and white film negatives.
A more complete "gray-card style" chart which Kodak also offers is a 21-step gray card. Exposing this card would allow you to plot your entire exposure curve (in film terms, a "D log E" curve). This may be the intended purpose of the three-card "white balance" set you mentioned--to get an idea of your curve's "middle" and extreme shadow and highlight exposure range.
Television engineers also use gray cards called "chip charts." It has 10 steps ("chips") of gray in two reversing rows. These cards are used to color-correct studio cameras, where a video engineer would "paint" each camera manually, adjusting individual RGB levels on a "remote-control unit" (RCU).
This may have been the where the idea of white balancing on a gray card came from. It still works, providing your gray card is completely neutral in hue, but most camera operators and video engineers just use a white card to white balance cameras these days. As an ENG television camera guy, I often white balance on just a white sheet of paper. Premium-bond is very white. Even lower-quality papers these days still tend to be pretty darned white.
There are several companies that sell "white-balance" cards. Yes, they're white. Plus, they're laminated so they don't get dirty. They typically come with a few "warm" cards and a few "cool" cards. You would use the bluish card to force your white balance a little warmer; use the orange-ish card to make your scene a bit blue-er.
Personally, I think they're kind of a waste of money, but some guys love 'em. Some company came by our offices and gave us a few sets. I keep one in my run bag at work, but I've actually never even used it. Having a direct-entry Kelvin temperature adjustment feature in your DSLR (as in the new Nikon D7000), would do the same thing, and would be my preferred method of adjusting white balance if a scene was just a little too cool or a little too warm under any of the preset color temperature settings. Color casts, however, are a whole different problem, and can often only be corrected (at capture) by taking a manual white balance at the subject's position.
Most important, is to white balance under the lighting conditions where your subject is located. Say you're at a night game: if you're in the grandstand, and you would like to white-balance to the lights on the field fifty yards away, ideally, you would need to find something white on the field (however, if both the lighting and the surrounding surfaces are nearly identical to where you are, then a white balance from a card at that position would be fine).
Often, however, the problem is that your subject is under different light sources, or under a differing mix (proportion) of sources (of varied color temperatures), and/or is surrounded by different-colored reflective surfaces, compared with the sources/surroundings in your immediate shooting position.
