My initial reaction is to say that you should plan your novel as a trilogy. If you plan out the trilogy beforehand, the books will seem more cohesive, and if you don’t like the idea of the trilogy, it is much easier to cut down a plot than it is to make up a believable twist on the spot.
Let’s take a deeper look into the pros and cons, though.
The pros to “winging it” and not planning it as a trilogy in the beginning is that if you end up running out of steam mid-novel, you will have the majority of your world building and character development already finished. There will be less pressure to push on and possibly end up with a finished product that you don’t particularly like. If there is one aspect of your story you are particularly drawn to, you have the option of letting the plot end there until you have the resources to expand on the world and character development whenever you wish. If you decide to stop after one book, you will have a self-contained story with the option of continuation. If your writing style tends to be succinct, then you won’t feel forced to expand your story over several novels.
The cons of “winging it” is that there is less motivation for you to spend time elaborating on specific parts, because you may or may not continue your series. Everything needs to be somewhat restricted and contained into one novel just in case something happens and you decide to end the story at one book. You will have to cut back greatly on the lore you planned on explaining in the first book because there just simply might not be any room or use for it in the future. Character development will have to be more abrupt, the depth of your world may be shallower, and there just generally would not be enough “page-estate” to add a significant amount of detail.
The pros of planning the trilogy are that you have the freedom to expand and draw character development out for however long you can imagine. There is no set timeline of when you need to have certain points wrapped up, as long as it happens before the final climax. You would have much more space to play around with your characters and mold them to their final selves. You say that there is a lot of lore you want to develop during the course of your plot, and by planning the trilogy out beforehand, you can choose exactly when these developments happen. It is much simpler to spread years of plot over the course of multiple books than it is to try to cram it all into one.
The cons of planning are that if you decide you don’t want to continue, you will leave piece of unfinished work. If you decide to publish and then run out of steam, you may leave fans hanging about the end of your storyline. There is a lot of pressure to finish a story than has been planned out in three novels. If you tend to struggle to meet word lengths requirements on essays, having to elaborate on the world and character development will be more difficult because you will need to be able to spread out your arcs.
It’s your choice as to what direction you take when writing. It depends on your comfort level, writing style, and ultimately commitment level.
I hope I answered your questions in full. Please follow up if you still have questions.