As Dan said, it depends on the ARF. I thought it would be a good idea to start with not only an existing airframe, but one built by someone else so I had something that I knew would fly and any issues would be my piloting skills. However, I ended up buying a plane that needed servos, an Tx/Rx, battery, ESC, and a motor. That was a lot for me to figure out on my own as a newbie. Not only did I accidentally order the wrong size servos, but I got a motor that was completely wrong. I didn't follow the horrible directions and gave the plane too much throw and I had a terrible experience. Which is why Dan said that an ARF can be a bad idea. But it really depends on what you have to add to it. If you really just need a battery, ESC, and a Tx/Rx, then the ARF Bixler is a great way to start. (It's the motor and setting up the servos/throws that can be tricky.)
As the others said, there are perks to buying a RTF plane, and perks to scratch building. As Mr Clean said, you don't have to pick just one. You can buy a plane and build a plane. It's a mistake to think that one plane will last you a very long time. Mostly because it's nice to have options and try different planes to see what type you like the best and so on.
As far as 3ch vs. 4ch, I've heard people say that they wished they'd just started with 4ch and that the 3ch are just a waste of time. I had problems with the 3channels I tried to start with because I kept putting too much throw on them and over-correcting. Without ailerons you can't properly correct turning too sharp and you end up crashing. But a 3channel is fine, you just have to be more careful about your throws and not turning too sharp.
Lastly, a simulator is a great investment. Some Txs can be hooked up straight to a computer and I have RC Desk Pilot (
http://rcdeskpilot.com/) which is a free sim (for Windows only). It's not the best sim in the world, but it still greatly helps with orientation. Which is the biggest thing to learn. The sim basically does two things, it takes the "oh crap, what does this stick do again" and the "oops, I turned the wrong way" out of flying. Which are the two biggest things to learn. (If your Tx can't be hooked directly to a computer, you can get this:
www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=43335 I plan on buying one.)