A flat universe refers to a theoretical model in which the universe has a flat geometry, meaning that the curvature of space is zero. This is one of the possible shapes that the universe could have, along with a closed universe (positive curvature) and an open universe (negative curvature).
A 3-dimensional universe means that it has three spatial dimensions, which we experience as length, width, and height. This is the most common understanding of the universe and is based on our observations and measurements of the world around us.
Our understanding of the universe as 3-dimensional comes from a combination of observations and mathematical theories. For example, the laws of physics, such as gravity and electromagnetism, are based on the assumption that the universe has three dimensions. Additionally, measurements of the cosmic microwave background radiation support the idea of a 3-dimensional universe.
If the universe was not 3-dimensional, it would mean that our understanding of physics and the laws that govern the universe would need to be revised. It would also have significant implications for how we perceive and experience the world around us. For example, if the universe had more than three dimensions, we may be able to observe and interact with dimensions beyond our current understanding.
Yes, there is a significant amount of evidence supporting the idea of a 3-dimensional universe. This includes observations of the cosmic microwave background radiation, the behavior of particles at the subatomic level, and the laws of physics that govern our daily lives. However, there are also theories, such as string theory, that suggest the possibility of additional dimensions beyond the ones we currently perceive.