how to prepare a letter in Word on an A4 sheet of paper, folded in two places. i.e. having 3 parts. as in the figure below.



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See if you can find a template.
Three-fold brochures are ordinarily done with Text Boxes that may be linked, one for each of six panels.
Download a few templates and see if any will meet your needs.
If not, study their design and create your own.

If you're folding the letter in 3 to fit inside a DL envelope and you are using a window envelope the only thing you need to consider is the placement of the address.

The method I have used is remove the film from the window, place a folded blank sheet inside the envelope, and mark the edges of the window on the paper. By measuring the distance from the top and left of the sheet you can then find where the address needs to be placed.

If you are not using a window envelope, then it really doesn't matter where you place the text elements.

Following up on Italophile's remarks, the simplest way to place text in one spot is using a Style that has a Frame as part of its definition.
Creating Frames in Word
Take a look at the Envelope Address Style for an example.