How do we understand the Hadith: "Be firm, for there is no one on you but a Prophet, a Siddeeq, and two martyrs," knowing that the second martyr is Uthman (may Allah be pleased with him), even though Uthman did not attend the Battle of Uhud because he was nursing his wife (may Allah be pleased with her)?

Uthman ibn Affan, may Allah be pleased with him, is one of the two martyrs mentioned in the Hadith (definition). He was absent from the Battle of Badr due to his wife, the daughter of the Messenger of Allah, may Allah's peace and blessings be upon him, being ill. He received the reward for being present at it and his share of the spoils. As for the Battle of Uhud, Uthman, may Allah be pleased with him, witnessed it. The Prophet, may Allah's peace and blessings be upon him, may have said the Hadith "Uhud is a mountain that loves us, and we love it" during the Battle of Uhud or at other times, because Mount Uhud is located north of Medina, and the Prophet, may Allah's peace and blessings be upon him, used to pass by it. He prayed over the martyrs of Uhud and stated that he saw them and would bear witness for them, and he feared for his Ummah the competition for worldly possessions.