You could prompt for the values using ASK fields and use REF fields to the associated bookmarks to insert the values in your calculation.
Better still you could create a userform with three text fields (or three comboboxes) to collect the three values, which would enable you to provide error handling for inappropriate values, and use the output of the userform to provide the values for your calculations.
At its simplest see -
http://www.gmayor.com/Userform.htm and attached template (create a new document from the template).