The 3 days following Eidul Adh-ha are known as the days of 'Tashreeq' because they used to cut the meat in strips and leave them to dry in the sun. [Fat-hul Bar

And just like the day of 'Eid, fasting these 3 days is also forbidden; because the Prophet ﷺ said: ((أَيَّامُ اَلتَّشْرِيقِ أَيَّامُ أَكْلٍ وَشُرْبٍ، وَذِكْرٍ لِلَّهِ عَزَّ وجل)) “The days of Tashriq (i.e. the 11th, 12th and 13th of Dhul Hijjah) are days of eating, drinking and remembrance of Allah, the Most Great and Glorious." [Muslim (1141)]