when at early game 3v3 you should be passive at FB and get in a hit with your disintegrate(Q).
when minions spawn you will always be the one with the most need of going top. be top until you are lvl 6 and if some other teammate need to be top too let him. because now you got tibbers! use ur abilities until your next ability stuns, then stun with dintegrate, summon tibbers, use incinerate and moltenshield after that for minions. 90% of the time you will take all damga in 2 seconds. just be aware of that if you want kills with annie, then wait until they don't have all HP. many people find themselves using the full combo and the enemy having 10% hp left and you get ksed.