I will agree with Galastel, and add the following.
If I were a plotter and intentionally writing a trilogy, I would provide some treatment (say 3 or 4 thousand words) of what is in the second book, and the third book. I would work on these treatments, to show the plot and arc and set in my mind what the big problem is in the second book, and the third book, and how the characters carry forward. I would not get too detailed, I would not get into the emotions and evolution of the characters other than rough characterizations. eg Two fall in love, or have a child, or lose a child, or break up. Note if people die, or new characters join the team. Note if the POV is changing from book to book, etc.
But then I would write the first book, review it, polish it, and try to get it published. The treatments you write, or plot outlines or whatever you wish to call them, are available to your agent/publisher. They probably DO NOT want to publish the trilogy all at once; publishers like to drop a new book a year. They want your first book to generate an audience they can work with (and if it doesn't, they don't want the rest of your trilogy). They want your second book to sell to that audience quick, and increase the size of your fan base. The same with your third book.
Dropping all three books AT ONCE will cost them three times as much, reduce the money they have for marketing, and is more likely to produce a financial bust.
The treatments that you write, after a publisher has already decided they like your style, and after they know you can finish a book satisfactorily, will make them more likely to pick up your first book and buy first rights to your subsequent books in the series. ("First" rights because if you give them crap for the second book, they won't publish it.)
Then you have a year for each subsequent book, and hopefully money to eat some lunch while you do. If it takes you less than a year, all good. You will be busy visiting bookstores to promote your first book. If a website is up, you may be answering fan email to promote your book.
Finish the first book, while the story is fresh in your head, to the point where you can read it through without cringing or frowning at anything. Finish your treatments. Then try to sell the book.
On Editing: At least for me, that is several run-throughs, six or seven to be honest. You will need the discipline to not embellish just because you thought of something to add; these drafts are not for more fun in writing, but to correct things that readers will be confused by or notice missing or think are weird or out of character or don't make sense. If you just keep adding to scenes (or adding scenes) for the fun of it, you will end up with something too long to publish!
Note that this is not a rule against adding anything, some awkward transitions, dialogue, or action sequences DO require adding sentences or even paragraphs to achieve clarity. Sometimes unclear description needs adding. The rule is against adding something that is not necessary to the plot, character development, or the reader's understanding of the scene, just because you enjoy embellishing on the scene.