If I have now PR3 how much increase should I expect next time if I got some backlinks from a blog having PR8 and some blogs who have PR4 and 5?
That depends on how many links there are on the pages with those particular PageRanks. Remember, PageRank is for PAGES, not sites. Just because a blog or site's home page has a PR 7 doesn't mean the entire blog/site does. There could easily be pages within it that have a PR of 1 or 2.