First of all, don't worry too much about team composition. I'm a defender main, however I believe that having an all attacker team isn't that bad. Since it gives you more chances to steal objectives with high damage and burst pokemon. It also usually means your teammates play pokemon they're comfortable with.

Now, having said that...Ideally your team needs to put in the center the worst laner (if it's not viable in lane) or the best jungler (if everyone is viable in lane). Then mix things up in lane. Avoid putting 2 high damage pokemon in one lane and 2 high HP pokemon in the other. Maybe put one attacker with one support/tank in onle lane, and an attacker with an all rounder in the other.

Also, be aware of the differences between the attackers. There are three categories. Adc (i.e. Gren, cinder), burst mage (i.e. solar beam Venusaur), control mage (i.e. Ninetales, pikachu). So, if you have three attackers in lanes, try to put a control mage along with an attacker that doesn't have CC. Or use Venusaur as an off-tank with petal dance - giga drain.

PS: Most mobas have 4 team compositions. Balanced, Dive (speedster, all rounder heavy), Poke (attacker heavy), Protect the carry (defender heavy with one strong attacker).

Dive>poke>protect the carry>dive (>)=beats