The solar spectral irradiance (SSI) data set is a key record for studying and understanding the energetics and radiation balance in Earth’s environment. Understanding the long-term variations of the SSI over time scales of the 11-year solar activity cycle and longer is critical for many Sun-climate research topics. There are satellite measurements of the SSI since the 1970s that contribute to understanding the solar variability over Solar Cycles (SC) 21 to 24, with most of these SSI measurements in the ultraviolet and only recently in the visible and near infrared for SC-23 and SC-24. A limiting factor for the accuracy of the previous results is the uncertainties for the instrument degradation corrections. Analyses of the past SSI data sets have identified some irradiance offsets and some small residual instrumental trends. These corrections are applied and then combined with a previous SSI composite data set, called the GSFCSSI2 composite, to provide a new SSI composite, called the LASP GSFC SSI #3 (or SSI3). This improved composite extends the wavelength coverage down to 0.5 nm and up to 1600 nm and the time coverage up to 2020. The solar variability results from the SSI3 are consistent, of course, with the observations from which are used to create the SSI3, but they do differ with some solar variability models, in particular at longer than 900 nm. The development of the SSI3 composite also clarifies the importance of overlapping missions for studying the 11-year solar activity cycle, particularly for wavelengths longer than 200 nm.