3 is like a semi-powerspike for Fizz, giving you the posibility to follow up a gank from your jungler or maybe just proc your Electrocute.

6 is your biggest power spike giving you the possibility to roam or all in your enemy laner.

What you wanna do in early game is trying to get as many minnions as you can and get 6 as soon as posible. The champion allows you to follow ganks from your jungler or help him to get the scuttler at level 3. Try to not be very aggresive in lane at level 1 or 2, you can start poking or going for trades at level 3.

There is only one skill order I consider good on Fizz and that is R>E>W>Q.

For the people who doesn't already know this, Fizz is a 100% roaming Midlaner. So what you really want to do is get 6 as faster as you can, push the lane hard under the enemy tower then look for a gank on botlane or toplane.