They are similar but not the same. To do something “under protest” usually means that you believe that, according to the law or the rules, you should not have to do the thing — but you do it because you are at risk of punishment if you don’t. To do something unwillingly usually means you were physically forced or otherwise pressured to do it. To do something reluctantly means you did it of your own free will, but weren’t happy about it and you wish that it could have been avoided.

Hi Jacob. Thank you very much for the lucid explanation.

1. "under protest" means that he did not agree with the fine and complained about it. To protest something means to take action or statement against something that expresses disapproval. 2. "unwillingly" means that he was against paying the fine but had to do it anyways for one reason or another. 3. "reluctantly" here means the same as "unwilling", but the sentiment is not as strong.