SQL0203 - Name xxxx is ambiguous, when using JOIN USING

I'm getting SQL0203 - Name MYKEY is ambiguous
with the following:

select mykey, flda, fldb, fldc
from tbla
join tblb using (mykey)
join tblc using (mykey)
^---cursor positioned here...

I've tried adding parens (in various places), but get the same error.

CREATE TABLE QTEMP/TBLA (MYKEY CHAR (10 ) NOT NULL WITH DEFAULT,
FLDA CHAR (10 ) NOT NULL WITH DEFAULT)

CREATE TABLE QTEMP/TBLB (MYKEY CHAR (10 ) NOT NULL WITH DEFAULT,
FLDB CHAR (10 ) NOT NULL WITH DEFAULT)

CREATE TABLE QTEMP/TBLC (MYKEY CHAR (10 ) NOT NULL WITH DEFAULT,
FLDC CHAR (10 ) NOT NULL WITH DEFAULT)

Running 7.1 TR6