These types of helicopters have gained alot of popularity since last year and its really no surprise. Everyone enjoys jumping on board the newest fad and why not. Tricopters are supposed to be fairly easy to use but of course they do take a fair amount of practice as with anything you do, practice becomes a skill. It is on my agenda to eventually build a tricopter as they tend to have a low parts count and they tend to be really stable which is a bonus for aerial photography and videography. If you are looking to build your own you can go to Rc Explorer. On that site you will find instructions on what you will need and also how to go about it. However, if you would rather just buy a quad copter you can look at the Turbo Ace X720 Quad which is offered at Wow Hobbies and the price range is from around 500 dollars to about a 1000 dollars.
Here is the Tri-copter that you can learn how to build from Rc Explorer :
Here is a cool video from Copter Kids and the success they have had with their Gaui quad-copter :
Hope this helps.