Private Sub ComboBox3_Change()

Dim wbSource As Workbook, rngSource As Range
Set wbSource = Workbooks("клиент.xls")
If Not ComboBox3.Value = "" Then
    Set rngSource = wbSource.Sheets("TDSheet").Range("A1:A100").Find(ComboBox3.Value)
    If Not rngSource Is Nothing Then
        TextBox4.Value = rngSource.Offset(0, 1).Value
        TextBox5.Value = rngSource.Offset(0, 2).Value
        '...
    End If
End If
End Sub

Private Sub UserForm_Activate()
Workbooks.Open Filename:="D:\Мои документы\Desktop\проект\клиент.xls"
'Workbooks.Open Filename:="D:\Мои документы\Загрузки\проект\клиент.xls"
ComboBox3.List = Workbooks("клиент.xls").Sheets("TDSheet").Range("A1:A100").Value 'клиенты
    
End Sub