Uthman was absent from Badr only because he was married to the daughter of the Messenger of Allah ﷺ and she was ill. The Prophet ﷺ said to him, "You will have the reward of one who witnessed Badr, along with his share of the spoils."
إنما تغيب عثمان عن بدر، فإنه كانت تحته بنت رسول الله ﷺ، وكانت مريضة، فقال له النبي ﷺ: إن لك أجر رجل ممن شهد بدرا وسهمه.