The correct answer is option 3: Heating of a platinum crucible.

A physical change refers to a change where the substance or material undergoes a transformation in its physical properties but does not change its chemical composition. In the case of option 1 and option 2 (burning of coal and wood, respectively), these are examples of chemical changes as the coal and wood are being converted into new substances through combustion.

On the other hand, option 3 involves the heating of a platinum crucible. Heating a platinum crucible is an example of a physical change because it only affects the physical state or properties of the platinum. The platinum does not undergo a chemical reaction or change in composition during this process. The heating causes the metal to expand or contract, but it doesn`t result in a different substance being formed.

Option 4, the heating of potassium chlorate, is also an example of a chemical change, as heating it causes it to undergo decomposition, breaking down into different substances.

Therefore, the only option that represents a physical change is option 3, the heating of a platinum crucible.