I read that my Canon Speedlite 430EXII could only be used as a slave on my new 5D Mark 3 and that I should buy a new flash. My flash seems to work on the camera, but since I'm such a new user with Mark iii and still figuring it out perhaps I'm not getting the best of it yet.
Should I buy something new?
THe 430EXII works fine with the 5D3 on-camera. All the features are supported, including controlling the flash settings through the camera menu (as opposed to on the flash directly, which is also supported).
The "slave" element you are referring to is as follows: when you want to use several flashes, one on the camera and one or more off-camera, the flash on the camera sends out a series of commands via infra-red[1] to the remote off-camera flashes. This is the "master" flash, and the remote flashes are the "slave" flashes. The 430EXII can't work as a "master" flash. What this means is that, if you have some other flashes off-camera, you can't put the 430EXII on your camera and control the remote flashes. You need one of the 500EX-series flashes to gain master functionality (550EX, 580EX, 580EXII), or else the ST2 transmitter.
Bottom line: if you want a single flash on-camera, the 430EXII is 100% compatible with the EOS 5D3.
[1] note that the 600EX flash uses radio signals instead, but that isn't compatible with 4xx and 5xx series flahes, so I've ignored it here.
I read that my Canon Speedlite 430EXII could only be used as a slave on my new 5D Mark 3 and that I should buy a new flash.
That's simply wrong. Any Canon EX-series speedlight (and many third party flashes) will work just fine with your 5Dmk3 provided you use them on camera, or off camera with the right cable. Your 430EXII is a great flash -- it's powerful with plenty of useful features, and moderately priced.
The thing that your 430EXII can't do is to control other Canon speedlights using Canon's master/slave protocol. Since you currently have just the one flash right now, that's not a problem.
Eventually, you'll want to move your flash off camera, and maybe even use more than one flash at a time. Canon's system makes that quite easy. There are cheaper alternatives, but if you decide to stick with Canon's system you'll want to pick up a flash that can work as a master. These are mostly the top of the line (or previously top of the line) flashes (600EX, 600EX-RT, 580EX, 580EXII, 550EX). However, it turns out that the diminutive (and cheap!) 90EX can also function as a master on bodies that are modern enough to have on-camera speedlight controls (including your 5Dmk3). So, for around $50, you can easily use your 430EXII off camera. That adds a lot of creative control (when you're ready for it) at a pretty reasonable price.
Obligatory link: If you're starting to get interested in flash photography, go read Strobist: Lighting 101. (Careful, you might get hooked.)
