How to obtain values from other columns in excel ??

Hiii all,
Plllease helpp. what is the method to concatenate values from two columns??
I want to obtain the values from three different columns in excel and want to check if those values are concatenated and present exactly in the fourth coulmn .

For eg.If column 1 has-PT50
column2 has-456
and column3 has -abc

in column 4 must contain PT50456abc.Otherwise itmust be colored in red. can we use '&' for it.? Please find the code.Its showin error.

Sub newone()

Dim res As String

For i = 4 To 8

res = "PT50" & Cells(i, 3).Value&Cells(i, 5).Value&Cells(i, 15).Value
If Cells(i, 16).Value <> res Then
Cells(i, 16).Select
    With Selection.Font
        .Color = -16776961
        .TintAndShade = 0
    End With

End If


Next i
End Sub

Help