ActiveMdiChild - call current class function

I have 4 windows forms : frmMain - set to mdicontainer
frm1,frm2 and frm3 (all 3 have the same function save(), update(), delete(); but the code is different for all)
---CODE---------------------------------
' not full code, just example
Public Class frmMain
      Dim doc As Form
      sub frm1_Click()
        If Not Me.ActiveMdiChild Is Nothing Then
            ActiveMdiChild.Close()
        End If
            doc = New frm1
        doc.MdiParent = Me
        doc.Show()            
      end sub
      sub frm2_Click()
        If Not Me.ActiveMdiChild Is Nothing Then
            ActiveMdiChild.Close()
        End If
            doc = New frm2
        doc.MdiParent = Me
        doc.Show()      
      end sub      
      sub frm3_Click()
        If Not Me.ActiveMdiChild Is Nothing Then
            ActiveMdiChild.Close()
        End If
            doc = New frm3
        doc.MdiParent = Me
        doc.Show()      
      end sub            
      sub save()
            'should call the current class's save function
            doc.save()  <--- Error "save() is not a member of 'System.Windows.Forms.Form'
      end sub            
end class
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Any help?