The Italian-built San Marco 3 was a cooperative space effort between the Italian Space Commission (CRS) and NASA. The primary objectives of the mission were:
- To provide density, neutral composition, and temperature data describing the equatorial upper atmosphere at altitudes of 200 km and above
- To measure variations in these parameters as functions of solar and geomagnetic activity.

A secondary objective was to determine the neutral density by using three independent measuring techniques. The spacecraft was a 75-cm-diameter sphere. Four 48-cm antennas protruded from the top of the sphere, for command and telemetry transmission. The structure of the spacecraft formed an integral part of the drag balance experiment. A light external outer shell was connected by a series of elastic arms to a heavier internal framework. Thus, from changes in the flexible arms connecting the two structures, atmospheric drag (and therefore density) was determined. Other onboard experiments included an omegatron mass spectrometer that directly measured the density and temperature of molecular nitrogen and a Neutral Atmospheric Composition Experiment (NACE) that directly measured the density of molecular nitrogen, molecular oxygen, atomic oxygen, argon, and helium. Unlike the earlier San Marco spacecraft, San Marco 3 employed an attitude control system, and a spin rate control system. In addition, solar panels were mounted equatorially on the inner core. The satellite performed nominally after launch until vehicle reentry on 28 November 1971.