How it works is that the manga, as mentioned, puts out three chapters a month. On the other hand, the One Piece anime is consistent about releasing four episodes a month. While this is the case, the manga is still decently ahead of the anime. For the most part, the manga is typically around 40 to 50 chapters ahead of the anime. This means that the manga is at least a year ahead of the anime. Events that are happening right now in the manga will not likely occur in the animated series until about a year from now. The reason for this is that manga is the source material. There needs to be enough time for Oda to create more series chapters. As such, the anime is almost always behind the manga. Even if it somehow gets close to catching up, filler arcs will begin to buy time. We have seen this happen many times throughout the long-running anime series.