Why do Earth and two other planets have atmospheres?

Earth, Venus, and Mars all possess atmospheres, though of vastly different compositions and densities. Planets can retain atmospheres when their gravitational pull is strong enough to prevent atmospheric gases from escaping into space and when they receive sufficient volcanic activity or other outgassing processes to create and replenish atmospheric gases. Earth's atmosphere formed from volcanic outgassing early in its history and was later transformed by biological processes, particularly the oxygen released by photosynthetic organisms. Venus retained a thick atmosphere due to its substantial mass and extensive volcanic history, though its proximity to the sun created the extreme greenhouse conditions we see today. Mars, being smaller and more distant from the sun, has a much thinner atmosphere because its weaker gravity allowed much of its early atmosphere to escape, and it lacks the plate tectonics and volcanic activity to replenish lost gases.