JOIN with Subquery?

Here is my query:
SELECT B.SHNAME, B.SHADD1, B.SHADD2, B.SHCITY, B.SHSTAT, B.SHZIP FROM FILESIZE.PICKHEAD AS A
LEFT JOIN FILES.SHIPMAST AS B ON B.SHIP# = A.SHIP AND B.SHCUS# = A.CUST
WHERE A.ORD# = 270068

Gets me what I need, however, there is the occasion where A.SHIP with = 0, which means that I need to go to a 3rd table. How do I add the logic to say A.SHIP = 0 (SELECT NAME, ADD ..) and not the B.SHNAME

Here is what I ended up doing.

SELECT A.THSHIP,
CASE WHEN A.THSHIP <> 0 THEN B.SHNAME ELSE C.CUNAME END AS NAME,
CASE WHEN A.THSHIP <> 0 THEN B.SHADD1 ELSE C.CUADD1 END AS ADD1,
CASE WHEN A.THSHIP <> 0 THEN B.SHADD2 ELSE C.CUADD2 END AS ADD2,
CASE WHEN A.THSHIP <> 0 THEN B.SHCITY ELSE C.CUCITY END AS CITY,
CASE WHEN A.THSHIP <> 0 THEN B.SHSTAT ELSE C.CUSTAT END AS STAT,
CASE WHEN A.THSHIP <> 0 THEN B.SHZIP ELSE C.CUZIP END AS ZIP
FROM FILESIZE.PICKHEAD A
LEFT OUTER JOIN FILES.SHIPMAST B
ON B.SHIP# = A.THSHIP AND B.SHCUS# = A.THCUST
LEFT OUTER JOIN FILES.CUSTMAST C
ON C.CUST# = A.THCUST
WHERE A.THTIK# = 326911