I’ve been planning a book for a while and I always planned on it being a series but now I’ve been thinking that maybe I should make it a stand alone. It’s basically this big competition that had three stages and each stage would’ve been a different book. But part of me thinks that maybe I should just put all of that into one book. Cause I dunno if that would be easier to have one.
A story that has three parts is not a series. If it is long enough to be split into several volumes it is a trilogy. If not, it is just a novel that internally is divided into three parts. In a series, each book can be read on its own, and while there may be some overaching developments readers usually don't have to have read earlier books to enjoy the latter.
For a three part story, there are two factors to consider:
What length and structure does your story have.
What sells better in your market segment, series or standalones.
The first consideration is the more important one. If your story spans several thousands of pages, it will commonly be broken into several separately published volumes. If your story can be told within 120 pages or so, it will be published in a single volume.
This is not something you have to think about when you write your book. Your agent and publisher will discuss this with you when you submit your manuscript. They may also suggest to expand your story or delete parts of it to make it more coherent or interesting. Rewriting is a common and large part of the publishing process. Few books are published as submitted.
The second consideration – what your market wants – is one that should come instinctively with your familiarity with your genre and audience. You will have read many books in the genre that you are writing in, and being a part of your audience (i.e. as a voracious reader of your genre and part of that community), you will know what works best and your ideas will automatically fall within genre conventions. It shouldn't be something you have to consider.
If you are unsure if what you write fits your genre, you either aren't familiar enough with that genre (and need to read more), or you aren't writing in the genre that you think you are.
All that said, a common problem is when aspiring authors don't have an idea about a story but begin with an idea about a style or structure. If your idea was to write about "this big competition that had three stages and each stage would’ve been a different book", then you are in trouble, because you begin with a structure and now have to come up with a story that fits that structure. The result will invariably be awkward and patchwork. If, on the other hand, you have an idea to write about an athlete who wants to participate in a competition and has to deal with whatever obstacles he faces, then the structure will naturally and without any effort on your part result from that story. And it will either be in three parts or it won't.