You need to fully qualify all your Range and Cells inside the With WS statement, by adding the . as a prefix.
E.g. instead of pattern = Cells(10, 2) use pattern = .Cells(10, 2) , the .Cells(10, 2) means Cells(10, 2) of WS , which is being advanced in your For Each WS In ThisWorkbook.Worksheets.
Code
Option Explicit
Sub Deletrows_Click()
Dim WS As Worksheet
Dim pattern As String
Dim RowCount As Long, i As Long, j As Long
For Each WS In ThisWorkbook.Worksheets
With WS
pattern = .Cells(10, 2) ' delete row if found the word total in it
RowCount = .UsedRange.Rows.Count
For i = 2 To RowCount
For j = 1 To 3 'find the word within this range
If .Cells(i, j) = pattern Then
.Cells(i, j).EntireRow.Delete
End If
Next j
Next i
End With
Next WS
End Sub
Option 2: Instead of using two For loops, you could replace the 2nd For loop with the Application.Match function, to look for a certain value throughout the row.
Code with Match
Option Explicit
Sub Deletrows_Click()
Dim WS As Worksheet
Dim pattern As String
Dim RowCount As Long, i As Long, j As Long
For Each WS In ThisWorkbook.Worksheets
With WS
pattern = .Cells(10, 2) ' delete row if found the word total in it
RowCount = .UsedRange.Rows.Count
For i = 2 To RowCount
' use the Match function to find the word inside a certain row
If Not IsError(Application.Match(pattern, .Range(.Cells(i, 1), .Cells(i, 3)), 0)) Then '<-- match was successful
.Cells(i, 1).EntireRow.Delete
End If
Next i
End With
Next WS
End Sub
Edit 2:
Option Explicit
Sub Deletrows_Click()
Dim WS As Worksheet
Dim pattern As String
Dim FirstRow As Long, RowCount As Long, i As Long, j As Long
Dim FirstCol, ColCount As Long
For Each WS In ThisWorkbook.Worksheets
With WS
pattern = .Cells(10, 2) ' delete row if found the word total in it
FirstRow = .UsedRange.Row
RowCount = .UsedRange.Rows.Count
FirstCol = .UsedRange.Column
ColCount = .UsedRange.Columns.Count
For i = 2 To RowCount + FirstRow
' use the Match function to find the word inside a certain row
If Not IsError(Application.Match(pattern, .Range(.Cells(i, 1), .Cells(i, ColCount + FirstCol)), 0)) Then '<-- match was successful
.Cells(i, 1).EntireRow.Delete
End If
Next i
End With
Next WS
End Sub