Combustion is when we burn a fuel, it is a chemical reaction and an exothermic one (these reactions are those who give out heat). For combustion, we need hydrocarbon (which is combustion's fuel) and oxygen, the oxygen burns the fuel and leaves us with carbon dioxide, water and energy!!
Combustion's word equation is:
As for respiration, it is a type of combustion. Our cells get energy: glucose ( glucose is the fuel) to be able to live, this process is called respiration.
Respitation's word equation is: 
Photosynthesis is how plants get their "food" (energy) to keep on living, the substances it needs and gives away are th same as those in respiration, except they are inverted:
