S3_3-alpha3 is commit e5df4eaafcfd6d227d9c5d97034350a20f3dc1da in freeciv git.
Version number in that commit is 3.3.0-alpha3

Both freeciv-3.3 datafile formats and network protocol have been
have been frozen in alpha3. Alpha3 compatible datafiles will be
compatible with final freeciv-3.3. Alpha3 client/server will be
able to interoperate with final freeciv-3.3 server/client.