There might be a better way but give it the try:

NOTE: I'm using TabStrip control as it offers more flexibility.

Code:
Option Explicit

Dim myTabIndex As Integer

Private Sub Form_Load()
Dim i As Integer

    TabStrip1.Tabs.Clear
    For i = 1 To 5
        TabStrip1.Tabs.Add i, , "Tab" & i
    Next i

End Sub

Private Sub TabStrip1_MouseUp(Button As Integer, Shift As Integer, x As Single, y As Single)
    If GetTab(x + TabStrip1.Left, y + TabStrip1.Top) > 0 Then
        If Button = vbMiddleButton Then
            TabStrip1.Tabs.Remove myTabIndex
        End If
    End If
End Sub

Private Function GetTab(x As Single, y As Single)
Dim i As Integer
Dim tabLeft As Single

    myTabIndex = -1
    
    For i = 1 To TabStrip1.Tabs.Count
        tabLeft = TabStrip1.Left + TabStrip1.Tabs(i).Left
        If (tabLeft <= x And tabLeft + TabStrip1.Tabs(i).Width >= x) And _
            (TabStrip1.Tabs(i).Top <= y And TabStrip1.Tabs(i).Top + TabStrip1.Tabs(i).Height >= y) Then
            
            myTabIndex = i
            Exit For
        End If
    Next i
    
    GetTab = myTabIndex

End Function