Using your keywords on the
<h1> is a good and acceptable SEO optimization. You can also use these keywords on
<h2> to optimize even more your page.
After, if you use your keywords in your all
<h3> on your page, it's kind of dangerous because it may be considered as keyword stuffing by Google.
A good way of SEO optimization for a page can be this one (just an example):
<h2>What is breeding for cats?</h2>
<h2>Cats breeding ground</h2>
<p>There are a number of different places to <strong>breeding for cats</strong>.</p>
Try to apply variations to your keywords and not use exact match ones. Google appreciates variations and you can avoid keyword stuffing in many cases.
Otherwise, do not use
<h3> in a text, it's semantic not correct.
<h3> is used as heading like for
<h6>. If you want to show important words in a text to Google, use
<strong> tag instead.