Can it be harm for my site in manner of SEO if I will insert BR tag inside H1 tag?
<h1> keyword1 <br/> keyword2 <br/> keyword2 <br/> </h1>
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<h1>Ramones <br> <span>Hey! Ho! Let's Go</span> </h1>
If it’s appropriate in your case depends on your actual content (a heading listing three keywords is most likely not a good idea in the first place).
However, even for inappropriate uses of
br, there is no reason to assume that it would affect your ranking in Google Search.
Theres a chance you'll be in the bad books with Google. It all depends on what the resulting screen looks like to various users when you're done.
For example, if you create a super simple page where each character in the title (H1 tag) are at least an inch high in height and you use a couple of breaks (BR tags) inside it, then most of the content of the page will be below-the-fold. It's a bad experience if a user has to scroll on a page in order to see the content.
Also, I'd avoid the BR in H1 because it may throw some SEO tools off when you check your web page within them, thereby producing inaccurate or unwanted results.
If you need to make lists, you can use
<ul> and format the spacing and font styles using CSS. and put the list within your document. The H1 tag isn't meant for listing items, especially the way you are doing it.