Most multicell P7 flashlight I've read about do their batteries in series, not parallel and use a driver to convert the higher voltage down to ~3.6-3.7 volts. Some energy is lost in this process, but it is still beneficial (longer runtime and better regulation).
As I understand it, the 18650 is well suited for driving the P7 directly (unregulated, which means no electronics except perhaps a resistor is involved in controlling the curent or voltage that the LED receives), so doing a couple of those in parallel is an interesting idea. Personally, I would lean towards using them in series with a driver because this will give you a regulated light (which I would prefer in this application). An unregulated ligth will get dimmer as the battery drains. The light output from a regulated light will not change before the batteries are almost completely depleted.
You could check out the battery forum if there are anyone who has discussed 2 or 3 18650 's in parallel - or ask your question there.
Honk Kong based dealextreme.com (short: DX) or kaidoman.com (KD) have a multitude of P7 lights with 1, 2 or 3 18650 batteries. Quality is somewhat a lucky lottery, but if you get a good sample they're the most bang-for-the-buck. You can either use the lights as they are (strapped to your helmet or the handlebar) or customize your own battery package. Or pay more money for higher quality from another vendor. There's lots of reviews of these lights on the CPF forums, search and you shall find ;)
