In my (very) limited experience a 3 reel system works fine for sheep, but ultimately they WILL get out so you need stock fencing between them and anything you REALLY don't want them to get to (i.e. the neighbour's fruit farm, my cider orchard or the road). Lambs need a really low live wire, so 4 wires are better. To put it another way, electric 3 or 4 wire works well for me to sub divide fields that already have a good stock fence perimeter.