@DJC that's strange because :
P1 is GPIO
P2 is the JTAG header for the BCM2835 (the Broadcom SoC), and is thus also of no use for us mere mortals, its only used during the production of the board. Probably to program the serial code, and the MAC address I dare to guess.
P3 is,a similar JTAG header, but it is (only) for the LAN9512 LAN/USB-HUB chip, and there is no need to use it during production, only when debugging a device.
A picture please