Hi Ailis, actually they cannot hold your resignation letter without valid reason but I would recommend for you to sit down with your superior/supervisor. State how do you feel about work and how actually working 3 hours overtime make you feel sicks. You need to show to them as well, that you can finish your job for the day right at time as well. get those win win situation but if they're not listening, you can just send the resignation letter according to your offer letter. I wish you all the best.