Dim cel as Range
Dim rng As Range
dim iSheet as integer

    With Excel.Worksheets("Sheet" & iSheet)
        Set rng = Range("A3" & .Rows.Count).End(xlUp) 'range od prvog do zadnjeg u kojem ima podataka
        For Each cel In rng
            ComboBox1.AddItem Cells(cel, 1) ' dodaje u combobox1 sve celije A3, A4, A5... do zadnje
        Next cel
    End With