Or a Trilogy.
There's a few things you need to think about as you start on this grand adventure.
Does the series or trilogy have a plot and character(s) arc? If so, good. If not what are you waiting for? There needs to be characters and plot issues that span the whole set of stories. You can't just have three different stories and lots of different characters and call it cohesive.
Similar but opposite. You can't just write one long story and break it into three parts and call it a series either. Each book has to have its one separate plot and character arcs.
Not just the good guy either. the bad guy needs an arc as well as motivation for being bad.
Details! Draw a map, or the floor plan or what creatures and characters look like. Store these drawings, pictures on your computer or in a hardbound notebook of some sort. Nothing like having a character change eye color, or names or something else in the middle of the second book.
In that notebook or document, write names dates and details. don't rely on your memory to do it for you. I promise you it will fail you and you will be scrolling through your previous MS for information. Also keep the important details in this place as well.
Lastly, have fun. A series or trilogy gives you some freedom you don't have in a stand alone. More room to develop characters and large complicated plots. Just remember a little problem prevention saves you a lot of trouble in the long run.
