Frank Hamelly has a great article up at MSDynamicsWorld.com covering 3 way matching in the P.O./Inventory process. Frank does a nice job of explaining the topic and provides plenty of examples and T-Accounts.
Dynamics GP Three Way Match (Purchase Accruals)
Archived post from 2008. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.