Zond 3 was a 1965 space probe which performed a flyby of the Moon's far side, taking a number of quality photographs for its time. It was a member of the Soviet Zond program while also being part of the Mars 3MV project. It was unrelated to Zond spacecraft designed for manned circumlunar missions (Soyuz 7K-L1). It is believed that Zond 3 was initially designed as a companion spacecraft to Zond 2 to be launched to Mars during the 1964 launch window. The opportunity to launch was missed, and the spacecraft was launched on a Mars-crossing trajectory as a spacecraft test, even though Mars was no longer attainable.

Zond 3's lunar flyby occurred on July 20 with a closest approach of 9,219 km (5,728 mi),[1] approximately 35 hours after launch. 25 visible light photographs and 3 ultraviolet spectra of very good quality were taken of the lunar surface, beginning at 01:24 UTC and 11,570 km (7,190 mi) prior to closest approach and ending at 02:32 UTC and 9,960 km (6,190 mi) past closest approach, covering a period of 68 minutes.[1][6] The photos covered 19 million km2 (7.3 million sq mi) of the lunar surface.[7]

Zond 3 proceeded on a trajectory across Mars' orbit, but not at a time when planetary encounter would occur. These images were transmitted by radio frequency on July 29 at a distance of 2.25 million km. To test telemetry, the camera film was rewound and retransmitted in mid-August, mid-September, and finally on October 23 at a distance of 31.5 million km, thus proving the ability of the communications system. The subsequent transmissions were also at progressively slower data rates but higher quality. The mission was ended after radio contact ceased on March 3, 1966, when it was at a distance of 153.5 million km. It operated for 228 days, roughly equivalent to the time needed to survive a journey to Mars and exceeding that needed for Venus.