Hi Vindhyachal,
The acceleration can be got by either reading X, Y, Z output separately or use the vector sum. These all can reflect the acceleration on the the accelerometer.
Of course, the accelerometer can test the earth gravity. But you need to consider the noise and offset error of the part itself. After you compensate the offset, average long enough time to lower noise, the vector sum of the 3-axes accelerometer output should be the earth gravity in stable condition.
BR
Neil