What is difference between constants, readonly and, static ?

Constant

A constant member is defined at compile time and cannot be changed at runtime. Constants are declared as a field, using the const keyword and must be initialized as they are declared. For example;

public class MyClass {   public const double PI = 3.14159; }
Changing the value of const variable will give compile time error.

ReadOnly
A read only member is like a constant as it represents an unchanging value. 
The difference is that a readonly member can be initialized at runtime.
Which cannot be do in const variable.
For e.g.
public class MyClass {   public readonly double PI;     public MyClass()   {     PI = 3.14159;   } }
Static
This means that the member can be accessed without creating an instance of the class.
o access a static class member, use the name of the class instead of a variable name to specify the location of the member. For example:  int i = Car.NumberOfWheels;