Code:
you need a MDIForm and two MDIChild Forms

in MDIForm

Private Sub MDIForm_Load()
    Form1.Show
    Form2.Show
    fctSetParentWindow Form2.hwnd, MDIForm1.hwnd
End Sub

in a module: 
Option Explicit


Public Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" (ByVal hwnd As Long, ByVal nIndex As Long, ByVal wNewLong As Long) As Long
Public Const GWW_HWNDPARENT = (-8)

Public Function fctSetParentWindow(frmChildHwnd As Long, frmParentHwnd As Long) As Long
    On Error Resume Next
    
    fctSetParentWindow = SetWindowLong(frmChildHwnd, GWW_HWNDPARENT, frmParentHwnd)
    
    If Err Then
        '
    End If
End Function
Best will be if you use a normal (not MDIChild) form and use the same function: fctSetParentWindow Form3.hwnd, MDIForm1.hwnd

Regards Xdream