Note that you are not required to use lockers in the pieces and that these could have been non-lockers.
There are three additional pieces to this puzzle. The first is that the pieces must be good. In other words they have to be placed appropriately to achieve their full point status. The second is that the rock that the pieces are placed in must be good. If the rock quality is poor, you may have to subtract points. And the third piece of the puzzle is that due to weird rock, flaring cracks or a lack of pieces that fit properly, you may not be able to build a 12-point anchor with three pieces. The rock may force you to use four, five or even six pieces.
There are times in the alpine when it is not possible to build a 12-point anchor. In this circumstance you may only be able to fashion a four or six-point anchor. To do this, place a piece or two and then tie them off to your harness. Once you're tied in, you can use your body as a supplement to the anchor and then belay directly off your harness with a tube style device.
You should only build your body into a 12-point anchor if you need to do so for speed on a very big objective, or you cannot build a system that meets or exceeds 12-points. When you belay off your body it is difficult to escape the belay if anything went wrong...and if you're anchor is terrible, then a belay escape isn't really an option anyway.
There is only one magic bullet when it comes to building a good anchor, and that's experience. The concept of a 12-point anchor will provide you with a good foundation for anchor building, but to really feel confident, you're going to have to build a lot of anchors...
--Jason D. Martin