joining 3 tables

I have this code WHICH IS JOIN OF 3 TABLES, 

SELECT 
     [Provider] AS Publisher
    ,[Price_Rate]
    ,[Source_ID]
    ,Media.Date
    ,Media.Impressions
   ,Media.Clicks
  ,'0' AS [Opt-buyers]
  ,'0' AS [Completed buyers]
  ,Media.accounting
    FROM [dbo].[budget]  AS BUDGET

  LEFT JOIN 
  (

  SELECT CASE WHEN [Ad_Set_Name] LIKE 'tw_%' 
THEN'twitter' 
WHEN [Ad_Set_Name] LIKE 'IN_%'
THEN 'insta'
ELSE '?'
END AS Publisher
      ,CAST([Day] AS Date) AS Date
      ,SUM(CAST([Impressions] AS INT)) AS Impressions
      ,SUM(CAST([Link_Clicks] AS INT)) AS Clicks
      ,SUM(CAST([Amount_Spent__USD_] AS money)) AS Spend
  FROM [dbo].[twitter]
  Group by Day,[Ad_Set_Name]

  UNION ALL

  SELECT CASE WHEN [Site__PCM_] = 'acuits.com'
THEN 'acqt'
WHEN [Site__PCM_]= 'PulsePoint'
THEN 'plpt'
WHEN [Site__PCM_] = 'SRAX'
THEN 'srax'
ELSE [Site__PCM_]
END AS Publisher
,CAST(Date AS Date) AS Date
,SUM(CAST(impressions AS INT)) AS Impressions
,SUM(CAST(clicks AS INT)) AS Clicks
,SUM(CAST(media_cost AS money)) AS Spend
  FROM [dbo] [pcm]
  Group by [Site__PCM_]
,Date

  ) AS new_sources_budget
  ON BUDGET.Source_ID = Media.Publisher
  WHERE source_id IS NOT NULL

and I'm trying to join another table called Email to what's this code is currently providing, but I'm  having tough time passing thus far. the goal is to add this code 

SELECT 
    SUM(CAST(_Send2 AS INT)) AS [Email Sent]
  ,SUM(CAST(_Open2 AS INT)) AS [Email Open]
   ,SUM(CAST(Click2 AS INT)) AS [Email Click]
FROM [dbo].[behaviour]
  Group by _Send2,_Open2,Click2

any help will be appreciated.