A family of Microsoft relational database management systems designed for ease of use.
I'd use a text box instead of a label. You can then set the Control Source of this text box to
=Switch([field1]="A","Number1",[field1]="B","Number2",[field1]="C","Number3")
Even better, create a small lookup table, with the distinct values of field1 in one field (the primary key), and the corresponding text strings in the other field.
Create a query based on the original table and the lookup table, joined on field1 and the first field of the lookup table.
If field1 can be empty, double-click the join line and select the option to return all records from the original table.
Add all fields from the original table plus the second field from the lookup table to the query grid.
Use the query as record source for the report.
You can now use a text box without label bound to the new field.