• -- formatted original

    select distinct

    t1.ServerName,

    t1.[Application ID],

    t1.[Application Name],

    t2.VP1,

    T2.VP2,

    T2.SVP,

    T2.[DC2015 Move Group],

    T1.[System Type],

    T3.Line,

    -- defines 1 column

    AppServerSource = (

    Select -- returns 3 columns

    IsNull(t1.ServerName,t2.ServerName) as ServerName,

    IsNull(t1.[Application Name],t2.[Application Name]) as [Application Name],

    Case

    When t1.[Application Name] is null then 'Check'

    When t2.[Application Name] is null then 'Payorder'

    Else 'Both'

    End AppServer)

    from Table1 t1

    INNER JOIN Table2 t2

    on t1.[Application ID]= t2.[Application ID]

    and t1.Servername = t2.ServerName

    JOIN BIM T3

    ON t1.[Application Name]= t3.[Application Name]

    order by Servername -- this is ambiguous and will error - use a table alias

    -- alternative

    select distinct

    t1.ServerName,

    t1.[Application ID],

    t1.[Application Name],

    t2.VP1,

    T2.VP2,

    T2.SVP,

    T2.[DC2015 Move Group],

    T1.[System Type],

    T3.Line,

    ServerName = IsNull(t1.ServerName,t2.ServerName),

    [Application Name] = IsNull(t1.[Application Name],t2.[Application Name]),

    AppServer = Case

    When t1.[Application Name] is null then 'Check'

    When t2.[Application Name] is null then 'Payorder'

    Else 'Both'

    End

    from Table1 t1

    INNER JOIN Table2 t2

    on t1.[Application ID]= t2.[Application ID]

    and t1.Servername = t2.ServerName

    INNER JOIN BIM T3

    ON t1.[Application Name]= t3.[Application Name]

    order by t1.Servername

    “Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw

    For fast, accurate and documented assistance in answering your questions, please read this article.
    Understanding and using APPLY, (I) and (II) Paul White
    Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden