Why there are three units to measuring temperature?

Answer 1

Fahrenheit and Centigrate (Celsius), as well as others, were developed independently one of another. Fortunately a standard emerged eventually, in the sense that now most countries only use Centigrade (and Kelvin, see below).

The Kelvin scale was developed (on the basis of the Centigrade scale) when it became clear that there was a lowest possible temperature (called "absolute zero"). It made sense, then, to have a scale that started at absolute zero, and had no negative temperatures. This simplifies several calculations.