The LPC932 uses max 5uA in power off mode, an external cmos wake up circuit should be possible with abt 10 uA. Make the actual calculation etc code as short and sweet as possible (should be possible to do in abt 1 millisecond). 1 ms out of 15 minutes will convert the 25Ma full speed power to a very low average say 2uA. Now you need find a battery and power supply that can produce 17uA for 3 years. Again the LPC932 should be a good choice since it is VERY Vcc tolerant i.e. no Vcc regulator, straight from the battery. I would recommend a large cap across the battery to help with the short burst of 25mA.

Erik