[AccessD] another IIF statement question

I have 3 columns - if one contains a record the other two are blank; and I
an trying to combine them 3 in one column. I was able to combine contest
of 2 columns --

(IIF(NOT isNull([Ron Data mid].[SW_Date_Closed]),  [Ron Data
mid].[SW_Date_Closed],[OW_Date_Closed])) AS W_Date_Closed

but i am not sure how to add the third.

i needs to be in the following fashion -
if (a is not null), a, else if (b is not null), b, else c)

any hints ?

(IIF(NOT isNull([Ron Data mid].[SW_Date_Closed]),  [Ron Data
mid].[SW_Date_Closed],
(NOT isNull ([OW_Date_Closed]), [MSS_Date_Closed] )) AS W_Date_Closed,


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