Как объединить две, три и более таблиц на SQL?
примерно так
select `Date`, source, Campaign, Ad, SUM(Click), SUM(Cost), SUM(`App Install`), SUM(Purchase)
  from (
    select `Date`, 'source A' as source,  Campaign, Ad, Click, Cost, null as `App Install`, null as Purchase from table1
    union
    select date(`DateTime`), 'source B', Campaign, Ad, Click, Cost, null, null from table2
    union
    select `Date`, Source, Campaign, Ad, null, null, `App Install`, Purchase from table3
  ) t
group by `Date`, source, Campaign, Ad
Читайте про JOIN