If you want blanks rather than NULL, then:
CASE WHEN ALTotalUnits / TotalProfiledHours >= 3 THEN CAST(ALTotalUnits AS varchar(30)) ELSE '' END AS [Higher than 3 Weeks],
CASE WHEN ALTotalUnits / TotalProfiledHours < 3 THEN '' ELSE CAST(ALTotalUnits AS varchar(30)) END AS [Lower than 3 Weeks]
SQL DBA,SQL Server MVP(07, 08, 09) A socialist is someone who will give you the shirt off *someone else's* back.