Hi Aloka, In the Chinese understanding, dukkha is not one of the 3 seals of Buddhism. Some tradition puts it as the fourth seal. I think the reason is that anicca itself implies dukkha. The wiki for Chinese version of 三法印 is different than the English version in this aspect. Chinese version said that these 3 seals came from "Great Treatise on the Perfection of Wisdom" written by Nāgārjuna. If you click on the link to the three bullet points on the Wiki link, you will see that they lead to anicca, anatta and nirvana.