Young Writers Society

I've got this idea for a story and scenes written down here and there, though the thing is, the idea feels lie it should be in three books, and that writing the last book first is what I'm planing to do because the last book includes a lot of detail about the happenings in the first two books (The MC lost her memory and slowly regains it in the story.).
Also, In the first two books, two of the three MCs are to die.
Is this an acceptable practice?

The super short answer:
"Acceptable practice" in the actual act of writing is anything that gets the book written.

"Acceptable practice" in the plot is anything you can make work/get away with.

Good luck!

I wouldn't recommend it, mainly because you're probably going to change your mind about the ending twenty three times before you reach it, and if you write the third book first then you'll feel obliged to conform to it. Plots progress naturally. Just keep hold of your ideas for the third book, and use them as an end goal to keep you ploughing through the first and second.

If your third book includes most of the details of the first and second, have you considered just having one lengthy book instead of dividing it over a trilogy.

Just my opinion though. Rosey's advice sounds cooler.