VBA + SelectionChange event

Try this

Sub MergeUnmerge()

    For Each cell In Range("TrueFalse")
        If cell = True Then
            cell.Offset(0, 1).Range("A1:C1").MergeCells = True
        ElseIf cell = False Then
            cell.Offset(0, 1).Range("A1:C1").MergeCells = False
        End If
    Next cell

End Sub