Splitting a power supply between two circuits
Use a 12V DC output AC Adapter to power the 12V module directly. You will then need a voltage regulator or DC to DC converter to reduce the 12V down to the 3V required for your other module. If the current requirement for the 3V module is low you could build a linear regulator out of an LM317, two resistors, two capacitors and a heatsink.

If your current requirement is more than a small number of mA the LM317 style design will get very hot and so a switching type DC-DC converter module would be required. You can find these for sale on eBay. Building your own switching regulator requires a certain amount of experience with such designs. Here is a sample type thing from eBay that could suit your needs. 



(image from above linked eBay auction page)

Here is a diagram of how the power lines from the 12V wall adapter would connect into your two modules.