'ambiguous column name' error refers to database, table, and column guuu
You have FROM sourcedb.answers and INNER JOIN sourcedb.answers so you are introducing the same table twice without giving either an alias.

Some of the join conditions appear probably wrong as well. 

 FROM   sourcedb.answers
               INNER JOIN sourcedb.questions_tests
                 ON sourcedb.questions_tests.testskey = '121212eczema'


doesn't reference sourcedb.answers anywhere in the ON. 

INNER JOIN sourcedb.answers_questions
                 ON sourcedb.questions_tests.questionskey = sourcedb.answers.questionskey


Doesn't reference sourcedb.answers_questions in the ON

I presume you actually need something like

SELECT answers.*
FROM   sourcedb.answers
       INNER JOIN sourcedb.answers_questions
         ON sourcedb.answers.answerskey = sourcedb.answers_questions.questionskey
       INNER JOIN sourcedb.questions_tests
         ON sourcedb.questions_tests.questionskey = sourcedb.answers.questionskey
WHERE  sourcedb.questions_tests.testskey = '121212eczema'