Tripindi Shradh pooja is performed for the unknown (अज्ञात पितरो), in order for them to attain moksha. If a person has pitra dosh, he/she can perform Tripindi (Brahma, Vishnu, Mahesh) Shradh in gaya. 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 in Gaya is being performed over the bank of the phalgu river in the for the shanti of the ancestors and to offer them sadgati ( the next stage of prosperity).