Drum: or knock out drum (KOD) is used when a small amounts of liquid droplets to be removed gas/vapor upstream.
Separator: separator is used when there is a quite a bit of liquid to be removed from gas/vapor upstream prior to delivering to the compressor suction.
Scrubber: srubber is used to separate sour gas in counter-current contact with the solvent/amine with internal containing trays and/or packings.
These are my understanding but all three terms are used interchangeably and can mean the same thing. It is to separate the Vapor-Liquid so they should be called as SEPARATOR as a common term, period!