Ok ..don't put pencils between the boards. If you want to impress me don't stack 3 boards together, get a solid 3 inch thick board and break it.
Spacers between boards or bricks do NOT make the break easier. They make the break SAFER.
When you break with spacers, the target breaks from the top, and if you fail, the energy you put into the strike will not rebound into your hand/foot/face/whatever.
When you break without spacers, the target breaks from the bottom, and a failed break is more likely to result in injury.
I was taught this as a kid, and discussed it at length with one of my kids who happens to be a mechanical engineer who breaks things (trains...) for a living. He confirms it.
As for 3 boards vs 1 thicker board...
A big factor in breaking is flexibility of the target. The more flexibility, the more difficult the break. This is one reason why I personally prefer to break bricks - they're more consistent from day to day, while boards get a LOT more flexible if you leave them out in the truck bed overnight while it rains.... Bricks are also cheaper.
Look at a leaf spring. Notice that they're not a single piece (generally speaking). Do you think there is a reason for that? Perhaps because multiple thinner layers increases flexibility?
Personally, I'm a fan of using good quality rebreakable boards. They're consistent, reliable, cheap (if you're using them regularly...) and better for the environment.