Is it compatible with D3100?
Yes, it should. Any digital Nikon lens should fit any Nikon digital camera. The lens mount on Nikon's hasn't changed in the last 20 years so the mount will always be the same. However, of course, you have to be careful of compatibility, auto focus, zoom, etc. In this case since the lens is digital and the camera is digital there should not be a problem. Since I don't have a D3100 and can only say it "should", I went to some reviews on B&P Photo's web page of reviews on the lens. There were 249 reviews. I did a search for D3100 and the found the following review.
FROM B&P Photo web page
"I would buy this product again
About Me Photography Student
I used this lens with my D3100 in weddings, sweet sixteen parties. It s has great auto FÖCUS . I have a 18-55 and a 70-200 and I constantly had to change lens but I can do a Lot of stuff without changing lens with this one.
Yes, I would recommend this to a friend
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I hope this helps and good luck with the lens