Fix coming later this month.
A bug in the June 6 3DS firmware update can cause Ridge Racer 3D to hang, according to a notice posted by Nintendo. After updating to the latest firmware version, backing out of Race Select or Machine Select in Ridge Racer 3D results in a frozen game.
Nintendo recommends that players simply avoid those actions, and rebooting if the system hangs. They will upload a new system update in the middle of June that fixes the problem.