Ok I'm targeting 3 long tail keywords, and so far i have one article which includes these 3 long tail keywords which are either underlined, bold, or italic. When i create new content should i continue adding these keywords and continuing this process? Or is there a limit of when you should stop?
No limit on it, you have to distribute the keywords naturally not by spam. Including the keyword in the article is good this will cause on increasing the density of keyword and this is good for SEO