Private Sub Workbook_Open()
    Dim wb As String
    Dim wbpath As String
    Dim NameA As String
    Dim NameB As String
    Dim NameC As String
    
    Application.WindowState = xlMaximized
    
    NameA = "Name_of_My_First_Workbook"
    NameB = "Name_of_My_Second_Workbook"
    NameC = "Name_of_My_Third_Workbook"
    
    wb = ActiveWorkbook.name
    wbpath = Workbooks(wb).path
    Dim xls As New Excel.Application
    
    xls.Workbooks.Open (wbpath & "\" & NameA  & ".xlsm")
    xls.Visible = True
    ActiveWindow.WindowState = xlNormal

    Dim xls2 As New Excel.Application
    xls2.Workbooks.Open (wbpath & "\" & NameB & ".xlsm")
    xls2.Visible = True
    ActiveWindow.WindowState = xlNormal
    
    Dim xls3 As New Excel.Application
    xls3.Workbooks.Open (wbpath & "\" & NameC  & ".xlsm")
    xls3.Visible = True
    ActiveWindow.WindowState = xlNormal
End Sub

Option Explicit
Dim xls(1 To 3) As Excel.Application
Dim wbk(1 To 3) As Excel.Workbook

Private Sub Workbook_BeforeClose(Cancel As Boolean)
    Dim blnSaveChanges As Boolean
    Dim n As Long
    ' amend to True if you want to save before closing
    blnSaveChanges = False
    
    For n = 1 To 3
        wbk(n).Close blnSaveChanges
        xls(n).Quit
    Next n
End Sub

Private Sub Workbook_Open()
    Dim wb As String
    Dim wbpath As String
    Dim NameA As String
    Dim NameB As String
    Dim NameC As String
    
    Application.WindowState = xlMaximized
    
    NameA = "Name_of_My_First_Workbook"
    NameB = "Name_of_My_Second_Workbook"
    NameC = "Name_of_My_Third_Workbook"
    
    wb = ActiveWorkbook.Name
    wbpath = Workbooks(wb).Path
    Set xls(1) = New Excel.Application
    
    Set wbk(1) = xls(1).Workbooks.Open(wbpath & "\" & NameA & ".xlsm")
    xls(1).Visible = True
    ActiveWindow.WindowState = xlNormal

    Set xls(2) = New Excel.Application
    Set wbk(2) = xls(2).Workbooks.Open(wbpath & "\" & NameB & ".xlsm")
    xls(2).Visible = True
    ActiveWindow.WindowState = xlNormal
    
    Set xls(3) = New Excel.Application
    Set wbk(3) = xls(3).Workbooks.Open(wbpath & "\" & NameC & ".xlsm")
    xls(3).Visible = True
    ActiveWindow.WindowState = xlNormal
End Sub