MYSQLを使用した複数のテーブルからのSQL結合データ

次のことを試してください：

select x.time, a.value as value_tba, b.value as value_tbb, c.value as value_tbc from (select time from a union select time from b union select time from c) x left join a on x.time = a.time left join b on x.time = b.time left join c on x.time = c.time