How do I go about boosting these longtails to the top 3?
1) create a few internal links (1-2 for start, increase to more than 2 if it doesn't work) with the long tail keyword as exact match anchor, but make sure that those internal links come from similar pages.
2) create new product pages with these long tail keywords as the title of the page and onpage-optimize that content with these long tails
Any of the 2 methods above will work, if not, find a way to use them both simultaneously
Do I add them within the product description?
oh, you don't have them in the product description yet! So how were you going to rank for them, then? Adding a keyword in the content is one of the 1st steps to ranking for that keyword, but since you don't even have them there... I'm actually surprised that you're ranking in top 10 for them anyway, although that might be due to a well-structured domain name with good authority and trusted by google. Many times google does reward sites that they trust with rankings for keywords that they don't target, based on the domain's authority and topical relevance of the domain to those keywords alone, so probably that's why you were ranking for those keywords in the first place.
Just add them in the content if you can fit them in there naturally
... And point 1-2 internal links with exact match anchors to them if needed....
Build more backlinks with the corresponding anchor text?
as a last resort, yes! But if you had a choice between building powerful links to rank long tails, or building links to rank high volume keywords I would definitely take advantage of the power of those links to rank for better keywords, especially since - by the way in which you've phrased your comment - it sounds like your domain is in google's good graces and you want to take advantage of this.
If you're some Elon Musk in disguise and have no issues throwing a few millions of $$$ at backlinks to rank long tail keywords then go ahead and do it! Otherwise, if your budget is limited, take advantage of your domain's authority and trust with google to rank for keywords with higher search volume. Just create product pages for those new keywords and point the strong backlinks at those pages instead of pointing them to meaningless long tails (also make sure that the new pages are indexed first, otherwise it might not work).
I notice that some of these kws are of a different search intent as well.
so? Search intent only matters when you create the content (because you want the content to address the intent of the keyword exactly), otherwise the intent doesn't matter because you still want to rank for those keywords (if you do want to rank for them that is...)
