This is a really good conceptual question from NEET point of view. It is from NCERT Biology Class 12 Chapter 2 ( Sexual Reproduction in Flowering Plants)

Double fertilization is a complex fertilization process in flowering plants  (angiosperms) where the fusion of the female gametophyte with two male gametes takes place. One sperm fertilizes the egg to form the zygote(2n) and the other sperm fuses with the two polar nuclei to form the endosperm(3n) i.e.  the triploid primary endosperm nucleus (pen) . This phenomenon of fusion of the two polar nuclei with one of the male gametes is know as triple fusion. As it fusion of three haploid nuclei, so it is know as triple fusion.

The nuclei involved in triple fusion are the two polar nuclei.