Actually the TREK 3 Series is an excellent bike for a beginner. It is a basic bike with a front suspension, mechanical disc brakes, and good enough components that is less than $600. It is perfect for a beginner to get his feet wet, enjoy bicycling, and gain experience. Once you have been riding awhile, you may feel the desire to move up to a better equipped bike, but by then you will have aquired the knowledge and experience to know what specific improvements you want your new bike to have over the TREK 3. And until then, you will have a good bike shop behind you to bail you out of any of the typical newbe problems that arise.
Also, don't be afraid to visit some other shops, look at other brands of bicycles, and test ride them. You might end up loving one of the TREKs or you might like a Giant or Specialized better. Try out a bunch and pick the one you like best. But all of the TREK 3 Series bikes are good bikes for the money.
You can find some awesome deals on the Internet, but only if you know what you are doing. An inexperienced cyclist can really get taken if they go shopping on the net. If you get a bike that is the wrong size or a used bike that is practically worn out, it may sour you to bicycling so that you end up taking up a different sport. Don't distrust a bicycle shop owner. Most bike shops are not going to rip you off. They want to get you on a bike that you will love so that they get your business back for repairs, tune ups, accessories, and finally another new bike when the time comes. If a bike shop rips you off on your first bike, then they lose all of the other potential business that they could do for you. Plus a happy customer usually brings in additional business through referals while an unhappy customer drives it away with warnings of a bad shop.