Macro to check lenght of characters and merge cells

Hello, guys!

I am trying to do a macro that checks if the length of a row is more than 3 characters, if that's correct - merge that field with another 45 into a third one. However, I really got lost so I'd need your help. This is what I got so far.

Sub MergeCells()
    Dim last As Long
    Dim   As Long
    Dim rangecells As Range
    Set rangecells = Range("M1:M100")
    Application.ScreenUpdating = False
    last = Cells(rangecells.Rows.Count, 1).End(xlUp).Row
    'loop
    For i = last To 1 Step -1
        If Len(Cells(i, 1)) > 3 Then
            ?????
            formatexpression = "=(M36&" "&N36&" "&O36&" "&P36&" "&Q36&" "&R36&" "&S36&" "&T36&" "&U36&" "&V36&" "&W36&" "&X36&" "&Y36&" "&Z36&" "&AA36&" "&AB36&" "&AC36&" "&AD36&" "&AE36&" "&AF36&" "&AG36&" "&AH36&" "&AI36&" "&AJ36&" "&AK36&" "&AL36&" "&AM36&" "&AN36&" "&AO36&" "&AP36&" "&AQ36&" "&AR36&" "&AS36&" "&AT36&" "&AU36&" "&AV36&" "&AW36&" "&AUX6)"
        End If
        Application.ScreenUpdating = True
    Next i
End Sub

Question is how can I specify these 45 rows to be merged with that "M1"?
Got really lost...