The fusion of three haploid nuclei during double fertilization is known as triple fusion.
The process of triple fusion takes place inside the embryo sac. As a result of pollination, pollengrains land on the stigma, they germinate and give rise to the pollen tube. In pollen tube, reproductive cell divides to form two male gametes. The pollen tube then enters releases the two male gametes in the embryothe sac. Out of the two released male gamete one gametes fuses with the nucleus of the egg. cell to from zygote. The other gamete fuses with the two polar nuclei present in the central cell. and forms the triploid primary endosperm nucleus hence it is called triple fusion.
Since in this process two male gametes fused with the female gamete and the secondary nucleus respectively therefore, it is also called double fertilization.