In this case, author entries are in the "last name, first name" format.

You can execute a command that takes the three first characters from the entry, changes them to upper cases, and saves them in the CALL data field.

UPDATE LIBRARY_NEW SET CALL = UPPER(SUBSTRING(AUTHOR from 1 for 3))

In this case, author entries are in the "first name last name" format.

You can execute a command that takes the three first characters from the "last name" part, changes them to upper cases, and saves them in the CALL data field.

Retrieves last name and saves into the CUSTOM2 data field:
UPDATE LIBRARY_NEW SET CUSTOM2 = SUBSTRING(AUTHOR from CHAR_LENGTH(AUTHOR)-(POSITION(' ' in REVERSE(AUTHOR))-2) for POSITION(' ' in REVERSE(AUTHOR)))

After reviewing values in the CUSTOM2, run this:
UPDATE LIBRARY_NEW SET CALL = UPPER(SUBSTRING(CUSTOM2 from 1 for 3))

Use the CUSTOM VIEW feature: you can define a SELECT statement to review results before you execute the SQL command that modifies data values. The SELECT definitions create and display a temporary table.

SELECT AUTHOR, UPPER(SUBSTRING(AUTHOR from 1 for 3)) AS CALL1 FROM LIBRARY_NEW