Data Field Question

Are you able to take Case statments that have AS (Field name) and use those to subtract after one another under the data tabin sql?

For example i created 3 case statments and they have 3 columns

SUM(CASE WHEN action = 'b' AND glsrc IN ('RO', 'RR') THEN glamt ELSE 0000000.00 END) AS RentalBilling,
SUM(CASE WHEN action = '6' THEN glamt ELSE 0000000.00 END) AS Depreciation,
SUM(CASE WHEN action = 'a' THEN glamt ELSE 0000000.00 END)
AS Repair

Under the data table I want to Subtract RentalBilling, Depreciation, and Repairs and then have that out come posted in a new column called totals.