What are the three necessary condition required for nuclear fusion to take place in the sun.

Nuclear fusion reaction can not occur on earth ordinarily. But can occur in the sun naturally but it requires certain condition which are : 

1. enormous hydrogen availability
2. extreme heat which is naturally present inside the sun 
3. presence of extreme pressure which enhances the fusion of two similar charged nuclei. 

The Sun is a main-sequence star, and, as such, generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 620 million metric tons of hydrogen and makes 616 million metric tons of helium each second.Fusion is the process that powers the sun and the stars. It is the reaction in which two atoms of hydrogen combine together, or fuse, to form an atom of helium. In the process some of the mass of the hydrogen is converted into energy.

it's fueled by nuclear reactions that fuse hydrogen atoms together into helium and other heavier elements, releasing huge amounts of energy in the process. In this way, the sun is slowly converting its mass into energy in the form of sunlight, and it has more than enough fuel to last billions of years.