Since all three operating systems are very different, it doesn't make sense to have identical hardware.

Harmattan is very much OMAP3, Windows Phone Snapdragon and Symbian is stuck with ARM11 chips.
ARM11 chips are the least expensive, followed by snapdragon and OMAP3. Dual Core chips are a lot more expensive, and are mostly advantages for multitasking, so that may be more of a tablet/pad kind of chip.