VBA to replace cell text that starts with ### to = in 3 different columns without affecting rows 1 to 15 - excel

Question

I'm not very familiar with VBA, but am in need of a macro that would replace ### to = for columns B, F, and J without affecting rows 1 to 15.
I found the below code as my starting point, but can't seem to adjust it to my situation, or if this would even work for my situation...
this code was replacing everything in B that was not empty to Title starting at B2
Sub replace_text
Dim lrow As Long
Dim rng As Range
lrow = Cells(Rows.Count, "B").End(xlUp).Row 'Detect last data entry row in Column B.
For Each rng In Range("B2:B" & lrow)
If rng <> "" Then
rng = "Title"
End If
Next rng
End Sub
essentially, my goal is to replace every occurrences of text strings that start with ### with = without affecting the remainder of the strings.
everything in column B, F, and J starting at row 16 would start with ### but do not have the same text after the ###.
other columns would also have ### at the start but would need to remain intact

Answer 1

Worksheet.Evaluate: Replace Text
Sub ReplaceText()
Const FIRST_ROW As Long = 16
Const COLUMNS_RANGE As String = "B:B,F:F,J:J"
Const BEGINS_WITH_STRING As String = "###"
Const REPLACE_STRING As String = "="
Dim ws As Worksheet: Set ws = ActiveSheet ' improve!
Dim rg As Range
With ws.UsedRange
Set rg = .Resize(.Rows.Count + .Row - FIRST_ROW).Offset(FIRST_ROW - 1)
End With
Dim arg As Range, aAddress As String, Formula As String
For Each arg In Intersect(rg, ws.Range(COLUMNS_RANGE)).Areas
aAddress = arg.Address
Formula = "IF(LEFT(" & aAddress & "," & Len(BEGINS_WITH_STRING) _
& ")=""" & BEGINS_WITH_STRING & """," & "SUBSTITUTE(" _
& aAddress & ",""" & BEGINS_WITH_STRING & """,""" _
& REPLACE_STRING & """,1)," & aAddress & ")"
'Debug.Print Formula
arg.Value = ws.Evaluate(Formula)
Next arg
MsgBox "Text replaced.", vbInformation
End Sub