How do they work when they get charged or fail to break an enemy unit so that the combat continues for another round? Does only the 3 models in the front get to fight or do they have to widen their frontage or something like that?

If you don't break them on the first round, combat continues as normal in later rounds with only those models in base contact able to fight.

As per the normal rules, if you won the round and are only engaged in the front, you may redress ranks.

You don't have to do any of this though, you can choose to fight on with only the three models.

So if they dont redress ranks, do they still get rankbonuses for those ranks in the back? And if they do, I presume they lose that rank bonus?

I think they keep the bonus during subsequent turns if they stay in lance formation. But if they redress ranks, they obviously lose the bonus.

Their rank bonus is calculated based on their formation.

If in Lance formation, use the Lance formation rules, and if redressed use the normal rules.

Ok, thank you very much guys :).