What will happen if I pay extra on some credit cards and default on one?

You have to manage all your credit cards properly. Pay the bills in full and within time. You will see positive changes in your score.

Depends on your credit utilization ratio. If it doesn't make much difference to your credit utilization ratio, then there won't be a big difference in your credit score. If your credit utilization drops, then your score will go up. If your credit utilization ratio increase, then your credit score will go down.

Defaulting on one card is always a negative remark. So, instead of paying extra than the minimum payments, at first, make sure you're able to make at least the minimum payments on all cards.
However, you should always try to pay the entire balance on every card at every billing cycle.

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Pay the bills in full and within time. If it is not possible, You must make minimum payments to all your cards.