Code:
Option Explicit
Private Declare Function SetParent Lib "user32" (ByVal hWndChild As Long, ByVal hWndNewParent As Long) As Long

Private Sub Command1_Click()
UnloadOtherForms
Form2.Show
SetParent Form2.hWnd, Frame1.hWnd

End Sub


Private Sub Command2_Click()
UnloadOtherForms
Form3.Show
SetParent Form3.hWnd, Frame1.hWnd

End Sub

Public Sub UnloadOtherForms()
    Dim frm As Form
    
    For Each frm In Forms
        If frm.Name <> "Form1" Then
            Unload frm
            Set frm = Nothing
        End If
    Next
    
End Sub