Well, no one here can tell what Google's specific criteria are - we can only give an educated guess. One thing I'm pretty certain about is that CTR is analyzed by various niches or silos. Maybe not the market niche (although that is also possible) but separated by the query type, user intention, universal search implementation, freshness - those kinds of factors.
Google just doesn't do "one size fits all" in any area. It's not in their DNA. There are too many PhDs in statistics on staff for that to happen!
what is considered a good CTR for first 3 positions?
The question is flawed (no offense!) as the first 3 organic positions could be anything from positions 1-3 to 8-10 in the results depending on image search, PPC, local map, news, youtube etc.
Even if you have a clean page where the first 3 organic results are the first 3 results given (which is incredibly rare), there are so many variations according to industry, commercial vs non-commercial, geo-specific queries and so on that any CTR average figure is just throwing numbers out for fun.
Long, long, long gone are the days where searchers and searches were predictable enough to say "a #1 position gets X% of clicks, a #2 position gets X% of clicks" and so on.