There is no way to do this as a paragraph style. My suggestion would be to simply set the first-line indent for the style to the location you would get with three spaces.

Using spaces for placement is a holdover from using typewriters.


4. Use Tables and Tabs to Arrange Text in Microsoft Word
3. Using Styles to Format Text

Would your publisher, instead, accept space between paragraphs, perhaps in a block style?

It might seem old fashioned, but that's what they want - 3 spaces. Apparently it makes it easier when they are feeding it into their desktop publishing program.