Is the amplitude of each wave (P, ORS, T) always the same in different cardiac cycles?

No the amplitude of each waves are different on different cardiac cycles.

Explanation:

The PQRST waves are used to represent the atrial and ventricular depolarization and repolarization.  These waves are reflected in ECG.  In this as the series, p occupies the first place then QRS complex and at last T wave. In the QRS wave complex,  R wave is said to be the largest wave.  The direction and amplitude of r wave depend on the electrical picture. The R wave represents the ventricles depolarization so it is the largest wave.