Other than mis-placed commas, reserved words as table names, the where clause 'problem' is solved by putting the expression in the where. General hint, don't use column names as alias names for an expression, you'll end up with a situation where you're not sure if SQL's using the column or the alias

DECLARE @Start DATETIME

DECLARE @End DATETIME

SET @Start = DATEADD(dd, -3, GETDATE())

SET @End = GETDATE()

SELECT a.ordernuber ,

b.product ,

CASE WHEN o.loadate >= p.loaddate THEN o.loaddate

ELSE p.loaddate

END AS ComputedLoadDate

FROM [order] o

INNER JOIN product p ON o.itemid = p.itemid

WHERE CASE WHEN o.loadate >= p.loaddate THEN o.loaddate

ELSE p.loaddate

END BETWEEN @Start AND @END