Excel VBA Userform Combobox - Load Multiple Columns Values

Example call joining column values

As close as possible to your original post you can read all values into a 1-based 2-dim temporary array, join 1st, 3rd and 5th column values by & and assign them back to the comboboxe's .List property:

Private Sub UserForm_Initialize()
    Dim ws As Worksheet
    Set ws = Worksheets("Tabelle1")
    Dim lastrow As Long, i As Long
        lastrow = ws.Columns("A").Cells.Find("*", SearchOrder:=xlByRows, SearchDirection:=xlPrevious).Row
    Dim v       As Variant
        v = ws.Range("C" & ws.Range("start_row_pu").Row + 1 & ":" & "G" & lastrow).Value2

        For i = LBound(v) To UBound(v)            ' join column values C, E, G
            v(i, 1) = v(i, 1) & " - " & v(i, 3) & " - " & v(i, 5)
        Next i
        ReDim Preserve v(1 to Ubound(v), 1 to 1)   ' redimension temporary array to 1 column 
        ComboBox1.List = v                         ' assign array back to .List property
End Sub

Further notes due to question in comment

After having assigned range data (e.g. C2:G4711) to a 2-dimensioned 1-based variant array v you have to loop through the 1-based array data now, where

• LBound(v) always starts from "row" 1 in a 1-based array (so callede lower boundary) and
• UBound(v) returns the upper boundary, e.g. 4710 (=4711 - 2 + 1 as starting from the second row) "rows";

now you refer to column C data in the original range via "column" index 1 of the variant array, i.e. v(i, 1), to E via index 3: v(i, 3), to G via index 5: v(i, 5). The above example joins the 1st, the 3rd and the 5th value in the array column items via the & connector and assigns the resulting string back to the first array column thus (over)writing the already read in data of range column C.

Eventually you have to redimension the original 5 array columns to only one in order to represent your wanted data row connected now by " - ".