I did not observe Sawm (fasting) for three days in Ramadan due to my illness. Can I make up for them at anytime?
You should make up for these three days you did not observe Sawm in Ramadan due to an illness during the days prior to the coming Ramadan, and the sooner the better. It is impermissible to delay any of these days until Ramadan if you are able to observe Sawm. If you delay them until next Ramadan, you should make up for them after Ramadan and feed a needy person for every day you delayed. (Part No. 10; Page No. 359) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.