SELECT *
FROM TABLE_NAME
WHERE Date_Column >= DATEADD(MONTH, -3, GETDATE())
Mureinik's suggested method will return the same results, but doing it this way your query can benefit from any indexes on Date_Column
.
or you can check against last 90 days.
SELECT *
FROM TABLE_NAME
WHERE Date_Column >= DATEADD(DAY, -90, GETDATE())