The Earth, the Moon, and the Sun appear round or nearly spherical due to the force of gravity. Gravity is a fundamental force of nature that attracts objects towards one another. When an object is massive enough, its gravity pulls it into a shape where the forces within it are balanced, resulting in a spherical or nearly spherical shape.
The force of gravity acts equally in all directions within an object. As a result, any irregularities or non-uniformities in the object's shape tend to be smoothed out over time as gravity acts to redistribute mass and bring the object into a more balanced configuration.
In the case of celestial bodies like the Earth, the Moon, and the Sun, their immense mass and gravitational pull cause them to assume a nearly spherical shape. Over time, the force of gravity compresses the material in these bodies, pushing it toward their centers and causing them to settle into a shape where the gravitational forces are evenly distributed.
It's worth noting that while these celestial bodies may appear perfectly round at a macroscopic scale, they can still exhibit variations and irregularities on their surfaces. Mountains, valleys, craters, and other geological features can create deviations from a perfectly smooth surface. However, these variations are relatively minor compared to the overall spherical shape of the bodies.