If I have an amplifier that provides +3dB, or an attenuator that provides -20dB, then how do I know if that applies to Voltage or power?

In other words, if I have a 0dBV signal and I run it though a +3dB amplifier, do I get 3dBV?

Similarly, if I have a 0dBm signal and it runs through +3dB amplifier, do I get a 3dBm signal?