I'm building a maintenance scheduling & tracking solution and am trying to create a field, which will calculate the date from the most recent entry into a related, LINE ITEMS table.  If I sort the related table by date descending, the following calculation works, as I've used it in other solutions:

GetNthRecord ( LINE_ITEMS::date ; 1 )

In this case, I'd like to filter the results by another field in the related LINE_ITEMS table.  For example, I'm going to have a text field, which contains a value from a list - MONTHLY, QUARTERLY, ANNUAL.  In the parent table, I'd like to have three calculation fields (one for each value), and obtain the (separate) most recent entry for MONTHLY, QUARTERLY, ANNUAL.

Do I need to have three, separate related tables, or can this be done via calculation "filter"?

If this is for display only, you could use 3 one-row portals, each filtered to show a specific type. Otherwise it gets more complicated.

---
BTW, instead of sorting the relationship and getting the value from the 1st record, you could have used simply:

Max (  LINE_ITEMS::date )

or (assuming the records are entered in chronological order):

Last (  LINE_ITEMS::date )

Note also that:

GetNthRecord ( LINE_ITEMS::date ; 1 )

can be written simply as:

LINE_ITEMS::date

since a reference to a related field always gets data from the 1st related record.