It's a moderately long, very uninteresting story! ;P
I was reading the Pokemon Special manga online (on a mobile phone, it was excruciating), and came across an issue that featured a fusion of Kanto's Legendary Birds, called Zapmolcuno.
I thought to myself, "That's awesomeeee, what if other legendary trios were fused?". At the time, Diamond and Pearl were the newest games, so naturally I viewed Giratina, Dialga and Palkia as the coolest trio. Hence the name, giradialkia. :P
People normally have no idea what the name is until I spell it out, then they're like "Ohhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh!" :P