As you probably know, a 2-way system divides high and mid frequency sounds
between 2 speakers, a tweeter and a mid-range. A 3-way system adds a "low-mid"
speaker into the mix, which helps to ease the work-load the other 2 speakers
have to deal with by handling certain frequencies itself. This also serves to
better blend sound between the main speakers and the subwoofer. A well designed
3-way system should be more accurate than a 2-way system. However, the benefits
may be lost because of your hearing issues. The downside to a 3-way system in
general is that it requires more space inside the car, and makes both
installation and system tuning more difficult. The best possible sound is often
achieved with a tri-amped 3-way system, where the tweeters, mids, and low-mids
get their own amps. This setup is extremely costly and space-consuming.

Who are some recommended online sources for receivers, amps, etc. ?

Crutchfield and Sound Domain are reliable online resellers, but I heartily
reccomend aganist buying a deck online. A big part of picking your ideal deck
lies in deciding which user interface you like the best. Alpine decks are among
the best made but I personally dislike their user interface and choose not to
buy one based almost entirely on that fact. Pioneer and Blaupunkt, IMO, have
the easiest interfaces with Eclipse (the brand I went with) somewhere between
them and Alpine. Picking a system is based on a sort of formula. You have to
weigh the kind of music you listen to, what you expect your system to do with
that music, how much you want to spend and what level of modification you are
willing to make to your truck. The solution to that formula will help you make
a short list of the products that fit your wants and needs and then you can
pick from among them.
Steve Grauman