What is the difference between double fertilization and triple fusion?

In angiosperms, double fertilisation refers to the fusion of one sperm cell with an egg (syngamy), and another sperm cell with the polar nuclei to yield a triploid endosperm (triple fusion).

 

 Double fertilisation involves two processes, triple fusion and syngamy.

Triple fusion --> Here, the haploid male gamete fuses with the secondary nucleus (diploid) to form the triploid endosperm nucleus, which later forms the endosperm.

Syngamy --> Here a male gamete fuses with a female gamete(egg) in order to give rise to zygote.

So triple fusion is a part of double fertilization.