Col “B” values should be match with Three months

Improving I still don't understand some of what you are doing and why so still hesitant to change too much but here is a simplified version of the macro that I believe gives correct results.  NOTE that 2 cases appear to be wrong based on your VLOOKUP but your VLOOKUP doesn't extend to the bottom on the Oct 2020 page.

Sub Matching2()
Dim row1, month3, month2, month1, Rating3, Rating2, Rating1, sku As String
Dim ws As Worksheet
Dim xa, xb, xc As Integer
xa = 1
xc = Worksheets("Main").Cells(1, 5)
Sheets("Main").Range("A:A").Clear
 
For Each ws In Worksheets
     Sheets("Main").Cells(xa, 1) = ws.Name
     xa = xa + 1
Next ws
Sheets("Main").Range("A2:C8").Sort Key1:=Sheets("Main").Range("B2"), Order1:=xlAscending, Header:=xlNo
For xb = 4 To xc

    month3 = Worksheets("Main").Cells(xb, 3)
    month2 = Worksheets("Main").Cells(xb - 1, 3)
    month1 = Worksheets("Main").Cells(xb - 2, 3)
    
    For x = 2 To 800
        If Worksheets(month3).Cells(x, 17) = "D" Then
            sku = Worksheets(month3).Cells(x, 2).Text
            If MatchFound(month2, sku, "D") And MatchFound(month1, sku, "D") Then
                Worksheets(month3).Cells(x, 18) = "Remove"
            Else
            End If
        Else
        End If
    Next x
Next xb
MsgBox ("Worksheets Updated")
End Sub

Function MatchFound(shtname, sku, val) As Boolean
   MatchFound = (Worksheets(shtname).Range("b:b").Find(What:=sku, LookIn:=xlValues).Offset(0, 15) = val)
End Function