Your catalogs on the old machine are presumably pointing to the location of the master image in a folder structure on the old computer. If you duplicate that image folder structure on the new machine, then all you need to do is copy the catalog from the old machine to the new. When you open LR3 for the first time, it will offer to create a new catalog or ask you to open an old one. Open the old LR2 catalog with LR3 and LR3 will then offer to convert it. When LR3 has finished converting the old catalog, you will have two versions of the same catalog. Once you have determned that the converted catalog is completed and all your images are where LR expects them to be, you can safely delete the old catalog from the new machine.
