Original text on this page previous to its migration to StructuredDiscussions:
IL-2: Forgotten Battles and IL-2: 1946 are the same game. 1946 is one of the many expansion packs released for "Forgotten Battles". Both Steam and GOG releases titled "1946" currently include Forgotten Battles with all of its expansions. Some of the expansions used to be available as standalone retail releases, but this is mostly irrelevant now.
Original 2001 IL-2 technically is a different game (which never got any expansions or updates after v1.2), but most of its content is included in current versions of FB/1946.
On Steam, Battle of Moscow is not a standalone game either, but rather one of the many DLC for Battle of Stalingrad.
On developer's website, Battle of Stalingrad and its expansions are released as "IL-2: Great Battles Series", and can be purchased separately. All of them are still identical from the technical side of things, so separate PCGW entries are not necessary.