"database" is a fuzzy term in FileMaker. Clearly you have 3 tables, A, B, and C. Are they in the same file or separate files? Are you using an older version of FileMaker where you can only define one table in each file?
In order for a field in Table B to copy (look up) data from a field in Table C. They must be linked by a relationship so FileMaker can determine which record in Table C will provide the value to be looked up.
If you have this table structure:
and LineItems are linked to products this way:
LineItems::ProductID = Products::ProductID
Then you can set a price field in LineItems to use a looked up value auto-enter option to copy the price from a record in ProductID that has the same productID number.
This is not the only way to "draw data" from another table. Other methods do not actually copy the data and will then update automatically when you update the data in the related table. Which method meets your needs depends on what you need to do with your database.
I am using 8.5. Yes different files.
My description fits your situation, you'll just need to use Manage | Database | Relationships to create relationships that link the tables. The fact that the tables are in different files means that you'll need to define an external data source reference to the other file, but once that is done, what I describe will work for you.
Where do I find Manage-Database-Relationships? Once I define the external data source will tha then populate the fields in the "dependent" file?
It may be named differently in your version. Check the File Menu. I don't have your version, but know that you have some means for defining a relationship between tables and that's what you need in order to make this work.
Got it working thanks.