Not the most elegant, but you could nest IIF statements... like...
Assuming
table 'tblDates' and
3 date fields, 'Date1', 'Date2', and 'Date3'.
Query could be:
SELECT
tblDates.Date1 AS D1
, tblDates.Date2 AS D2
, tblDates.Date3 AS D3
, IIf([D1]>[D2] And [D2]>[D3],[D1],IIf([D2]>[D3] And...