By default the Bypass Traverse Checking user right is assigned to everyone. With this right granted you don't need to specifically assign permissions to the folder1 or folder2, to allow the users to get to folder3. However, the user will not be able to browser to the folder, if they don't have list\read rights to these folders, they will need to be type the complete path in i.e. \server1\folder1\folder2\folder3.
Gary.