I wasn't sure what datatype U_Tax_year was so I left it as INT. However, this query should return the 3 months previous to the month you set.
DECLARE @start_month DATETIME;
DECLARE @start_year INT;
SET @start_month = '2013-04-01';
SET @start_year = 2013;
SELECT dbo.[@EIM_PROCESS_DATA].U_Tax_year
, dbo.[@EIM_PROCESS_DATA].U_Employee_ID
, SUM(CASE WHEN dbo.[@EIM_PROCESS_DATA].U_PD_code = 'SYS033' THEN dbo.[@EIM_PROCESS_DATA].U_Amount ELSE 0 END) AS PAYE
, SUM(CASE WHEN dbo.[@EIM_PROCESS_DATA].U_PD_code = 'SYS034' THEN dbo.[@EIM_PROCESS_DATA].U_Amount ELSE 0 END) AS TAXABLEPAY
, dbo.OADM.CompnyName
, dbo.OADM.CompnyAddr
, dbo.OADM.TaxIdNum
, dbo.OHEM.lastName + ', ' + ISNULL(dbo.OHEM.middleName, '') + ' ' + ISNULL(dbo.OHEM.firstName, '') AS EmployeeName
, dbo.OHEM.govID
FROM dbo.[@EIM_PROCESS_DATA]INNER JOIN dbo.OHEM ON dbo.[@EIM_PROCESS_DATA].U_Employee_ID = dbo.OHEM.empID CROSS JOIN dbo.OADM
WHERE dbo.[@EIM_PROCESS_DATA].U_PD_code IN ('SYS033', 'SYS034')
AND dbo.OHEM.U_Process_month IN (DATENAME(MONTH, DATEADD(MONTH,-3, @start_month)), DATENAME(MONTH, DATEADD(MONTH,-2, @start_month)), DATENAME(MONTH, DATEADD(MONTH,-1, @start_month)))
AND dbo.[@EIM_PROCESS_DATA].U_Tax_year = @start_year
GROUP BY dbo.[@EIM_PROCESS_DATA].U_Tax_year
, dbo.[@EIM_PROCESS_DATA].U_Employee_ID
, dbo.OADM.CompnyName
, dbo.OADM.CompnyAddr
, dbo.OADM.TaxIdNum
, dbo.OHEM.lastName
, dbo.OHEM.firstName
, dbo.OHEM.middleName
, dbo.OHEM.govID;