Not Updated For Current Season
This is a team I've tested 3 times, it did alwas work!
Great experience for 3 players
None survives if low
Many helpers in teamfights, and in escaping
Olaf will have to solo top or bot
Karthus, olaf and Warwick have to be caring about ganks (always call mias)
Karthus has to be really a good pusher in middle, and a fast exping
The team is divided in this way:
Olaf Solo (top or bot)
Singed and Morgana in the other lane
Well, in early game the first achivement to reach is to farm as fast as possible, that's for Karthus (of course) and for olaf, who will get duble experience for solo laning. Warwick will have to cover Olaf if recalling, but he shouldn't try to gank anyone before level 6.
The breaking lane will be the Singed-Morgana one. The combo is snearl+throw under turret (many people will complain about this, try not to do it too many times.
Karthus will tell you when he gets to level 6 and have his ulti ready, it's really important to tell him when to use him, a fed Karthus is really important for all the team, as he's really useful in team fights.
Warwick should get the Dragon every time it's possible, starting from level 7 for giving some gold to all the team.
When early game is up karthus and morgana should have some stacks on the Mejai's, so they should have a lot of AP. Olaf should be at the same level as Karthus, Warwick Singed and Morgana will be at the same level as them. Singed should start being a good tank so he qill be the battle initiator in team fights with his ulti on, his slow, and his Fling, Karthus will slow the rest of the team with the wall, Morgana snearls, WW and Olaf will do the rest like stuns from WW and slows from Olaf.
WW should care mostly of ganking, getting Baron at around level 15/16 and pushing turrets down.
Pushing with Olaf and WW won't be a problem, you can get a turret down in like 5 seconds. Just wait for WW to get the baron and try to get an Ace or nearly one, push and win!!
PS: in Singed's build i wuold substitute the Cataliyst with Tears of the Goddess, and then go for Manamoon. substitute one Sunfire with Rod of Ages, it will help you in getting down turrets faster.