How to size coupling and bypass capacitor for simple BJT CE stage
Without C2 the gain of the amplifier is approximately Rc/Re and, because you are adding C2 its low impedance shunts Re making the gain higher and sometimes considerably higher. At low frequencies that gain falls as the impedance of C2 rises and, 

at f = \$\dfrac{1}{2\pi R_E C_2}\$ the gain has back to near enough the dc gain of the circuit.

C3 limits low frequencies being developed across RL and from midband, as the frequency drops, the 3dB point is f = \$\dfrac{1}{2\pi R_L C_3}\$.

Similar story for C1 - as the frequency lowers the AC voltage at the base will drop and the 3 dB point will be approximately when f = \$\dfrac{1}{2\pi R_1||R_2 C_1}\$.