It is not possible to use Word documents inside of PrintShop Mail, the closest thing you can use are rtf files, but they wouldn't have the formatting that a word doc or pdf could have. Pdf is probably the best format to use. Then, you can create three layouts in PrintShop Mail, one for each type of letters, and then you place the variable data in the correct spots.

What you should do is open your Word documents, delete the places where you have variable info and leave a blank space. Then save this into a pdf file. Import the pdf in PSM, and then create text boxes and variables in the blank spaces, and fill them with the proper variable data. Do that for all three layouts. That should do the trick.