I didn't find your request real clear, but believe you are looking for
something like this.

Private Sub ComboBox3_DropButtonClick()
Dim str1 As String
Dim str2 As String
Dim str3 As String
Dim iPos As Integer
Dim rng As Range

str1 = ComboBox2.RowSource
iPos = InStr(str1, "!")
str2 = Left(str1, iPos - 1)
str3 = Right(str1, Len(str1) - iPos)
Set rng = Sheets(str2).Range(str3)
Set rng = rng.Offset(ComboBox2.ListIndex + 1, 0)
Set rng = rng.Resize(20, 2)
ComboBox3.RowSource = str2 & "!" & rng.Address
End Sub

Hth,
Merjet