Need help with a Calculated field with multiple IF statements

I have a list that I want to create a calculated field for.  I have a large lookup list that I'm trying to break up.  I have those working fine, as pictured below.

Now I want to create a calculated field called Active Lecturers 2.0 that will display the information from one of the 3 columns to the right in the screenshot.

The calculation I have now is working for the column: Lecturers A-M but it is still showing empty in the Active Lecturers field for the other two columns.

Here's the calculation I have so far:

=IF(
NOT(
ISBLANK([Lecturers A-M])
),
([Lecturers A-M]),
IF(
NOT(
ISBLANK([Lecturers N-R])
),
([Lecturers N-R]),

IF(
NOT(
ISBLANK([Lecturers S-Z])
),
([Lecturers S-Z])
)
)