Another tally table solution, but this one doesn't require the creation of a separate user defined function. CTEs are used to parse the "ARRAY" into a table which can be joined to "MyOtherTable" to support your insert.
DECLARE @input VARCHAR(2002)
DECLARE @array VARCHAR(2000)
DECLARE @sepchar CHAR(1)
set @sepchar = ',' -- separation character is a comma
set @array= '321,3AY,3LH,3PU,3PR,3SP,3JH,3VN,3VR,3VT,3VU,3VV,3VW,3VY,3UW,3WA,3WD,3WI,3WM,3WY,3WZ,3XB,3XJ,3XK'
set @input = @sepchar + @array + @sepchar -- begin and end with separation characters
;WITH tally (N) as
(SELECT TOP 1000000 row_number() OVER (ORDER BY sc1.id)
FROM Master.dbo.SysColumns sc1
CROSS JOIN Master.dbo.SysColumns sc2)
,ArrayAsTable AS
(SELECT substring(@input,N+1,CHARindex(@sepchar,@input,N+1)-(N+1)) as element
FROM tally
WHERE substring(@input,N,1) = @sepchar
and N < len(@input)
)
-- INSERT INTO MyTable (col1,col2,col3)
SELECT Element
FROM ArrayAsTable
-- JOIN MyOtherTable ON (whatever)
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