RAM with only one channel (the conventional, old type RAM slots) have a single path to and from the North Bridge (the controller on the motherboard handling the interactions between the CPU and the RAM, PCI, etc.). This means only one path is used for all the slots (whether the motherboard has 2 or 4, and this means that the speed is very limited. Dual channel however, means it has another path added to this controller, making read/write rates much faster. The same applies to Triple channel, with three paths.
This can be compared with a highway. The cars are the data packets, and the roads the "paths" or "channels" on the motherboard:
Only so many cars can travel on a highway with one lane. Add another lane however, and the capacity of cars increase. Another lane, and it increases even more.
But it is necessary to remember the speed at which the cars have to travel:
Having a lot of cars, only capable of 60km/h (compare this to DDR3-1333), makes the use of 3 lanes unnecessary, as they will only clutter up the road and not fully utilize it, and thus making the money it cost to build the extra road excessive.
But having the same amount of cars, capable of traveling at higher speeds of say 90km/h (compare this to DDR3-1800), you will gain much more if you build another lane or path for them to travel along, making the amount of cars using the road over a certain time justify the money needed to build another lane.
Now, you are at the max. You have still the same amount of cars, but now they are able to travel at 120km/h (lets make this the DDR3-2200). They will travel fast along one lane, faster along two and at maximum speed along three lanes. This means that it is worth the money spent to build another lane, making these cars give their full potential.
What I'm trying to say is this: You cannot say Triple channel is better than Dual channel, if you are only going to use a P4 processor. The same applies the other way around: Dual channel isn't always better, as overclocking on the new Sandy Bridge processors makes it possible to use both Dual and Triple channel to a lot of their potential.
Personally, I would've appreciated Triple Channel on my Asus P8P67, as I believe the i5 2500K processor I got can benefit from the extra little speed, but hey, the designers know best right?