Since, the sum of lengths of any two sides in a triangle should be greater than the length of third side.
It is given that two sides of triangle are 12 cm and 15 cm. Therefore, the third side should be less than 12 + 15 = 27 cm.
And also the third side cannot be less than the difference of the two sides. Therefore, the third side has to be more than 15 - 12 = 3 cm.
Therefore, the third side could have a length more than 3 cm and less than 27cm.