You could use a Word field coded as:
{EQ (NO \o (–,3))}
where the en-dash (i.e. –) is superscripted and the 3 is subscripted. You may need to increase the point size of the en-dash and/or decrease the point size of the 3. You can create the field braces (i.e. {}) pressing Ctrl-F9. Selecting the field and pressing F9 will update the field display, whilst Shift-F9 will expose the field code for editing.