The parallel processing of N1 and L3 elements of the 3L and 1M1 vehicles was being practiced when it was sent to the pad in November 1968. It was sent to the launch pad with the flight L-3 (L-1), '3L' payload stacked on top of the 1M1 first 3 stage N1. It was done to check out the finished flight control systems. Late in December 1968 (not in Corona series) the flight N1 first three stages of 3L was sent to the launch pad without the head L-3 payload to check out the engine heating systems.
Subsequently N1-L3, '3L' underwent its first flight test on February 21, 1969 at 09:18:07 UT and failed at 70 seconds. The second flight test vehicle 5L was launched July 3, 1969 at 20:18:32 UT and failed at launch, destroying the East "J" pad which took several years to rebuild. See other photo, this website. There are no full '3L', '5L', '6L', or '7L' photos in the Corona series.