Explorer 3 (Harvard designation 1958 Gamma) was an American artificial satellite launched into medium Earth orbit in 1958. It was the second successful launch in the Explorer program, and was nearly identical to the first U.S. satellite Explorer 1 in its design and mission.

Explorer 3 was the third satellite in the Explorer small satellite series, which started with Explorer 1, America's first artificial satellite. The Explorer program was a direct successor to the Army Ballistic Missile Agency (ABMA)'s Project Orbiter, initiated in November 1954 to use a slightly modified Redstone missile combined with solid-propellant rocket cluster upper stage to put a satellite into orbit.

In 1955, the "Stewart Committee", under the chairmanship of Homer J. Stewart of Jet Propulsion Laboratory (JPL), chose a Naval Research Laboratory (NRL) satellite plan using a rocket based on its Viking rocket (Project Vanguard) for the International Geophysical Year, which would start 1 July 1957. Nevertheless, ABMA hoped Redstone-Orbiter could still be used as a backup orbital system. Reentry tests that year conducted with the newly developed, Redstone-based Jupiter-C, further strengthened ABMA confidence in their vehicle as an orbital launcher.

Following the launch of the Soviet satellite Sputnik 1 on 4 October 1957, Project Orbiter was revived, with two shots authorized as a back-up to Vanguard in early November. The failure of America's first attempted Vanguard launch on 6 December 1957, cleared the way for an "Explorer" (as the crash program was dubbed) to be the first American satellite.

Working closely together, ABMA and JPL completed the job of modifying the Jupiter-C to the Juno 1 and building Explorer 1 in 84 days. An experiment developed for Vanguard by George Ludwig, comprising an Anton 314 omnidirectional Geiger tube detector for measuring the flux of high energy charged protons and electrons, was adapted for Explorer 1. Because of the high spin rate of the Explorer 1 rocket, the experiment's tape recorder had to be omitted, which meant that data could only be collected when the satellite was in sight and range of a ground station.

Explorer 1 took off 31 January 1958, becoming America's first satellite. Its Geiger tube worked properly, but acted contrary to expectations. As the satellite ascended in its orbit, the radiation count increased, then abruptly dropped to zero. When the satellite was descending, the tube abruptly began detecting charged particles again. As data could only be received about 15% of the time, it was yet impossible to determine the phenomenon Explorer had detected.

The objective of this spacecraft was a continuation of experiments started with Explorer 1. The payload consisted of a micrometeorite detector (a wire grid array and acoustic detector) and the same cosmic ray counter (a Geiger-Müller tube) experiment included on Explorer 1, but this time with an on-board tape recorder to provide a complete radiation history for each orbit, Ludwig having had time to accommodate for the spin-stabilization of the satellite.

Its total weight was 14.1 kg (31 lb), of which 8.4 kg (19 lb) was instrumentation. The instrument section at the front end of the satellite and the empty scaled-down fourth-stage rocket casing orbited as a single unit, to be spun around its long axis at 750 revolutions per minute. Data from these instruments would be transmitted to the ground by a 60 milliwatt transmitter operating on 108.03 MHz and a 10 milliwatt transmitter operating on 108.00 MHz.