Thuis guide is about Nasus in 3v3 setup. This champion has a passive that already gives lifesteal. so the idea of the champion is to hit as often as possible to get as much life back as you can, so try to stick close to the enemy.

I use Clarity and Heal

Clarity I use this spell so I dont need to get any Mana regen items. These will slow my tank items down and your team wont like that. I am also able to give my teammates mana if they need it. A lot of times I used it in teamfights when I saw that a teammate needed mana to make that last blow.

Heal This spell is brilliant at that start! Almost every 3v3 battle there will be a FB in the middle at the bottom. With Heal you get them in beleaving they have you! but then you just pop Heal and you kill 2 or 3 of the enemie. This actualy works!! and in teamfights its a brilliant spell. Every champion close by gets healed. Aint that great or what!

The item build is based upon the start with a Doran's Shield. It is a really nice item from the start since it gives a bit of armor, health and health regen.
for the next item you either get Ninja Tabi or you can go for Mercury's Treads this depends if the enemy has more stuns or more AD characters.

For the next item it is adviced to get Sheen. This item is perfect for Nasus' Siphoning Strike ability.

For the next couple of items you have to check the enemies setup just like with your boots. If they have more AD characters I suggest you get Sunfire Aegis first and then Force of Nature but most of the time I myself go for Chain Vest and then for Negatron Cloak, Make sure you go for Giant's Belt after that one and finish Sunfire Aegis This way I am good vs AD and AP at start. Thus in teamfights I can take more damage so my team can kill them.

Now I really want them to be slowed so I start off with getting Phage This item is brilliant for keeping your enemies close. and then finish the item off with Frozen Mallet. That way you have more health and again you can survive more so your team can nuke them down.

Most of the time this is when the battle is over, either we won or they won. But if the fight lasts any longer you can go and finish your Sheen and buy the Trinity Force and then a nice Guardian Angel. Again I have never been here so there might be better items to use when you play this long.