I play a ton Malzahar in 3v3s, I f you're new to him, be prepare to get owned in the face by brusiers for severl games. I can't tell you how, but try to be safe/passive. I'm always running around casting my spells an being super agressive, but you have to be careful you don't put yourself in a spot where you can't run away. Malz has no movement CC(excpt ult) so trying to run away from champs like singed is going to be a pain in the butt.
As for ulting, save it for ganks or when you and the taget have some breathing room. Just because when you ult, you're showing a "Darius ult my face" sign. You're silence wall, anti health pool and vision spells will be enough.
oh and p.s: Don't underestimate your pet, people do and it's awesome