Percy Jackson The Wrathful Demigod

Yo~ I have been struggling with writing all 3 books and one of my friends said something to me that I was thinking about a lot.

'Why write 3 book? Start with 1 and complete it, then continue to do it with the others.'

I decided to add on to that, update the other books while I focus on writing to completion one of my books. I am still a 14 year old newbie writer so I am asking for your opinion on this.