The church militant are God's people still living and fighting or struggling on Earth; the church penitent are those who died having asked for forgiveness but whose souls are yet to undergo Purgatory to expiate the sins committed while on Earth; and the church triumphant are those who lived heroic, saintly lives and died in the grace of God and whose souls have entered Heaven -- the saints.
These three church states are in fellowship with one another through PRAYERS: the militant asking for the intercession of the triumphant; the penitent asking for the prayers of the militant for them to finally enter Heaven; and the penitent praying for the militant that help pray for them.
Once the penitent church are released from Purgatory by the intercession of the militant church and enter Heaven, they become part of the triumphant church that will, in return, intercede all the more for the militant church.
The "communion of saints" simply means God's people fellowshipping with one another: the saints, the living and those souls still being purged.
