Nocturne is a great champ if played correctly, Once you start getting fed you are pretty much unstoppable, In a 3v3 you could focus getting fed and jungling while working the top lane. NO MATTER what your team says GO TOP!!! By time your lvl 18 everyone else should be around lvl 12 still. So this is when you get your team to help push and win.

In a 5v5 its a bit diffrent, Dont worry about going jungle let someone else jungle. You take top solo while they jungle and hope there is only one up top. You should be able to stay back and farm and take advantage of the top lane at all times. Easy thing to do is Cast Q, E, W, Ignite, as they run Cast R, If need be Flash to finish them off.

Notice the Attack Damage and the Crit rate and damage. This combined with some 24% lifesteal and your pretty much owning anyone.