Remove all 3 constants, as they're not needed in this short of code. A numeric value that is hard-coded isn't going to change. Now if you just used a double NAME, then it might change. So for the Temperature function:

void Temperatures(double fahrenheit) {
double celsius, kelvin;

celsius = fahrenheit * 1.8 + 32.0;
kelvin = celsius + 273.15;

printf("Fahrenheit: %.2lf\n", fahrenheit);
printf("Celsius: %.2lf\n", celsius);
printf("Kelvin: %.2lf\n", kelvin);

}