Hmm...My previous reply didn't post. Oh well...
The number three has special significance in lots of things. In mythology and fairy tales the number three is a common occurance. The 3 Little Pigs, Goldilock and the 3 Bears, The 3 Muskateers, etc.. It also has significance in religion. The Trinity, the three main gods of ? (I forgot the name of the religion!

) Shiva, Brhama and Vishnu. There's examples of the number 3 in Homer and in Shakespear, and although this may not relate to the significance of the number 3, many books are published in trilogys.