Probably because you should have been looking for UNION
instead of JOIN, e.g.:

(
   SELECT commonA, field1, field2,
       'constA3', 'constA4', 'constA5'
       FROM tableA WHERE ...
)UNION(
   SELECT commonB, 'constB1', 'constB2',
       field3, field4, 'constB5'
       FROM tableB WHERE ...
)UNION(
   SELECT commonC, 'constC1', 'constC2',
       'constC3', 'constC4', field5
       FROM tableC WHERE ...
);

you could use UNION ALL to preserve duplicates between
SELECTs - but I don't think that applies here.
Note that the column value's data types (or the "constant"
replacements) in each of the SELECT's column positions need
to be compatible.

'Any fool can write code that a computer can understand.
Good programmers write code that humans can understand.'
Martin Fowler,
'Refactoring: improving the design of existing code', p.15