I've made it simple because other than changing the table names I don't believe I've left anything out.

Here's a script to create/populate/select/drop. The select just needs to be finished. I don't know what else I could be providing as info. I don't understand why grouping by SubTotal makes sense.

CREATE TABLE A (ID VARCHAR(10), SubTotal DECIMAL(5,2))
GO
CREATE TABLE B (ID VARCHAR(10), POS INT, CSEQS VARCHAR(2))
GO
CREATE TABLE C (ID VARCHAR(12), TranAmount DECIMAL(5,2))
GO
INSERT INTO A VALUES('12345', '100')
GO
INSERT INTO A VALUES('12346', '100')
GO
INSERT INTO A VALUES('12347', '101')
GO
INSERT INTO B VALUES('12345', 1, '01')
GO
INSERT INTO B VALUES('12345', 2, '02')
GO
INSERT INTO C VALUES('1234501', 50)
GO
SELECT A.ID FROM A, C WHERE (A.SubTotal - SUM(C.TranAmount)) > 100
GROUP BY A.ID, A.SubTotal
GO
DROP TABLE A
GO
DROP TABLE B
GO
DROP TABLE C
GO