I'm not sure that G3 map can be loaded into basecamp, I downloaded my LakeVu HD G3 map on my computer, using GarminExpress, here are the files obtained:
006-D7248-09.gma
006-D7248-09.img
006-D7248-09.unl
006-D7666-06.img
006-D7667-06.img
Then, how to load this in BaseCamp I don't know, I tried to join with GMAPTool but without success, when I installed the "join" map in my BaseCamp the application crashed when loading the map.