Try DATEDIFF:
SELECT Incident_Number,
dateadd(s, submit_date + '19800' , '19700101') as 'submit_date',
dateadd(s, dtm_AlarmStartTime + '19800','19700101') as 'dtm_AlarmStartTime',
(submit_date - dtm_AlarmStartTime) / 60
FROM HPD_Help_Desk
WHERE DATEDIFF(mi, submit_date, CONVERT(date, GETDATE()) > 3