We have a large classified ads website with a good sitemap and robots.txt. We have good content google crawls us 4 times per second but.... we dont get indexed by Google and our new advertisement post appears in SERP after 2 or 3 days what should we do to get index in hours?
First, forget sitemap and
robots.txt. Those are fun, but not magic, they just help on a technical level for hard to reach content.
Second: Google doesnt work in "hours", Google works in "a few days". If you change something, it can take up to a few days, they're not waiting until you update something. For bigger/more popular sites this will be more often, but a 'normal' websites take a few days (Why do SO questions appear so quickly).
Then, I don't want to be rude, but if you appear in SERP page 2 or 3, you probaly don't have good content, because if you would, you would be on page one.
I recommend you use a few SEO score websites, there are free ones. Do a few of those. It might be worth your money to invest in a paid report as those give you much much more info.