Tripindi Shradh pooja is performed for the unknown (अज्ञात पितरो), in order them to provide moksha. If a person has pitra dosh, he/she can perform Tripindi (Brahma, Vishnu, Mahesh) Shradh in Prayagraj. It is performed when due to some reason, a person was unable to perform shradh for 3 continuous years. It is a ceremony of offering food and water to the souls of departed ancestors.
Tripindi Shradh Pooja in Prayagraj is being performed over the banks of the Triveni Sangam for the ancestors to offer them sadgati ( the next stage of prosperity).