Hi. This is my quick guide how do i play mordekaiser in 3v3. Sorry for my bad english and i hope, this guide will help you.

Flat HP... Why the hell secondary runes?
Because:
1) Huge early game bonus. Man, +181HP, thats like [Ruby Crystal]
2) Early game is almost most important phase for morde. You can farm very easy and stay on lane pretty long.
3) We dont build any HP items. This helps a little. :)

U don't have to follow my runes. I don't say it's the best choice, but as i said.. I like it =)

Okay. Masteries.
U DON'T NEED DEFENSIVE MASTERIES, U ARE TANKY ENOUGH.
U can run also Utility runes, but as i said already. I like early game domination and with Morde, if you farm early game, u just rape late game.

[Ghost] + [Ignite] because:
1) Ignite+Ulti= almost 100% kill
2) Ghost to chase, run away

U can replace Ghost with Flash, so you can flash through walls.

Pros:
Huge farming opportunity
No problems against anyone on solo lane
Big damage burst
Epic survival

Cons:
Quite squishy early game
Problems against some rangeds on solo lane
Farming does not go well
Bad ganker

Okay. This is most important thing. You can overfarm and overlevel enemy team so easy.
U need to get as many cs possible, but you are morde, don't worry.
In combination of your W,E,Q skills, you can stop minion wave without problem on lv4.

Your farming is easy. Just clear wave, let your minions push to their turret and run into jungle for more golds.. You can clear 1-2 camps before they push your lane, so you can return back and repeat. You can purchase oracle, so you can destroy enemy wards, since you are in jungle quite alot.