An element is the simplest chemical substance which cannot be broken down further using chemical methods. Sodium is an example of an element.
A compound is a chemical combination of two or more elements. The composition of a compound is always fixed. Water is a combination of hydrogen and oxygen in the ratio 2:1 by atoms. So, water is a compound.
A mixture is a physical combination of elements or compounds or both or even many mixtures. The constituents can be separated by physical methods. Each constituent shows its individual properties. Salt in water is a mixture because salt water has the properties of both salt and water and it can be separated by evaporation.