The temperature of objects is measured in degree Celsius by first assigning a temperature that has a value 0 C. A temperature lower than 0 C is expressed as a negative number and a temperature higher than 0 C is expressed as a positive number. The lowest temperature at which an object can exist is -273.15 C which is referred to as absolute zero.
If the temperature of a body is raised from 3 C to 6 C, its temperature does not become double. The reason behind this is the point on the Celsius scale at which temperature is fixed at 0 not being the lowest temperature at which an object can exist.
Consider a ruler used to measure distance in centimeter. If the 0 on this were shifted to the point 4, the actual distance between two points that is measured as 3 is 7 centimeters and when the ruler measures 6, the actual distance is 10. Even though the reading increases from 3 to 6, the actual distance is not doubled. This is similar to what is happening when temperature is measured in Celsius.
When temperature are measured in Kelvin, the 0 lies at absolute zero. In this scale the temperature at 6 K is double the temperature at 3 K.