I would also recommend ball valves on the bottom drain line and on the skimmer line to regulate the amount of flow for each line. Your bottom drain is going to out-compete your skimmer line so your skimmer will not skim the surface like it should. Also, it allows you to close off the pond for repairs or upgrades to your filters. Go with good quality valves especially the 3" one. Cheap ones become very difficult to turn.
Many have recommended these:
http://www.supplyhouse.com/Red-Flag-...ion-Ball-Valve
I use Georg Fischer valves. They're expensive, but you can find deals on eBay.