Cell colour change to match that to its left

There is no worksheet event that triggers on a format change (e.g. background fill). However, the code below achieves what you want, you just need to find a worksheet event that you are happy with.

Option Explicit

Sub t()

    Dim rColA As Range
    Dim rColB As Range
    Dim rColC As Range
    Dim rCell As Range
    Dim i     As Integer
    
    With ActiveSheet
        Set rColA = Intersect(Columns("A"), .UsedRange)
        Set rColB = rColA.Offset(0, 1)
        Set rColC = rColA.Offset(0, 2)
    End With
    
    For i = 1 To rColA.Rows.Count
        Set rCell = rColA.Cells(i, 1)
        If rCell.Interior.Pattern <> xlNone Then
            rColB.Cells(i, 1).Interior.Color = rCell.Interior.Color
            rColC.Cells(i, 1).Interior.Color = rCell.Interior.Color
        End If
        rColB.Cells(i, 1).Interior.Pattern = rCell.Interior.Pattern
        rColC.Cells(i, 1).Interior.Pattern = rCell.Interior.Pattern
    Next i
    
End Sub