If the Eastern Orthodox and Roman Catholic churches were united up until 1054, then why does the Eastern Orthodox canon of scripture contain 3 more books then the Catholic canon?
The RC canon was not finalized until Trent, long after the schism.
When was the EO canon decided?
Technically, it hasn’t been. There were some variations in the versions of the Septuagint that were used East and West during ancient times. Those 3 books were not included in the Western version for whatever reason. The canon of Scripture was officially declared in the West due to the issues with Protestantism which introduced doubt to the inspiration of the Deuterocanonicals. The East has not typically struggled with such issues. The council that declared the canon did not look at the additional books used in the East because they have not traditionally been used in the West. If they do, they may well declare their canon.
Actually this is not necessarily true. The first authoritive declaration of the canon was at the council of Rome in 382 under Pope Damascus. The second authoritative declaration occurred just a few year later at the council of Carthage in 419.
The council of Trent reaffirmed our canon in response of the claims by the Protestants that seven of the books in our Bible where not canonical.
The point that should be made is that the Orthodox churches have all the Catholic books with some additional books added. It also should be pointed out that up until recently all Catholic Bibles had a section that included the writings that where also found in Orthodox usage.
Now you asked why does the Orthodox churches have additional books added to their Bibles then. From my understanding it comes from understanding the original function of canonicity. For a writing to be canonical one of the requirements is liturgical usage, along with historical usage and orthodoxy. The canon as expressed in the West is the canon of Scripture where readings and passages from all of these books appear in our liturgy. In the East there was a few more writings used in their liturgies and as such they were not removed due to the historical liturgical function of these writings. In the Catholic faith both East and West, tradition is extremely important for it identifies the community and their relationship with God. So tradition is protected in all ways that are not a true conflict with Sacred Tradition.
An example of this occurred just recently when the Chalden Church was establishing a new liturgy. In this liturgy, they were going to use the full Nicene Creed with the clause “and the Son” added as it is used in the West. But Rome came back and recommended that they should keep the traditional form that did not possess the “and the Son” for it was more faithful to the traditions of that Particular Church.
Anyway this is just a simplified summary of a incredibly detailed subject but it should be a starting point for you.
Well, Eastern Catholics will have bibles with the EO canon of the OT and I have one in Ukrainian in my possession that has the full Orthodox Old Testament Canon with the “imprimaturs” of the Ukrainian AND Roman Catholic primates of Ukraine . . . How about that?
There are actually six more OT books in the EO canon with two more added in the Ethiopian canon. The Assyrian New Testament Canon has only 22 books. The Ethiopian NT Canon has 35, including the 8 books of the Apostolic Constitutions. The Celtic Churches often included the Shepherd of Hermas in their NT canon.
To me, this shows the ongoing, active role of the Church in establishing the canon of Scripture.