Sue, I would suggest one site. Pour your heart into that one rather than spreading yourself thin keeping up with 3. Spend your hosting budget on great hosting for one site vs. “ok” hosting for three.
Later, you always could put up smaller specific sub-niche sites that could attract folks who are interested in each sub-niche to your main site. Over time, you may even find >3 sub-niche topics for which you can create small “landing page” and free-report sites that drive subscribers toward your main / portal site.