The MacBook will run Lion with only 2 GB of ram, but it will be pretty slow, especially if you open a large application that needs memory. It will do a lot of swapping to the hard drive as it uses that as virtual memory.
If you go to the OWC or Crucial websites the memory kit for the MacBook calls for using a 4 GB module and a 2 GB module, no one makes 3 GB modules. Be sure to get quality ram certified for use in a Mac as Macs are sensitive to their memory. Users buying things like Kingston Value Ram, Corsair Vengeance Ram, PKNY and other inexpensive ram end up with problems and usually have to replace those modules. So, check what they have at OWC and Crucial and save yourself grief in the future.