How do I correctly group output from multiple tables?
So from what I can infer, your desired output is a column of all the dates from all 3 tables, with the 3 tables joined onto it.

So something like 

Select dates.date, tbl1_col, tbl2.col, tbl3.col from 
    (select distinct Date from table1 
    union
    select distinct date from table2
    union
    select distinct date from table3
    )dates --Now we have a list of all the dates from all the tables.
left join table1 on dates.date=table1.date
left join table2 on dates.date=table2.date
left join table3 on dates.date=table3.date
--we add the tables back on, keyed against the dates.


could be what you are looking for. Just an idea, reading into how you have phrased it.