I have read
(here) that the D3100 prevents use of 3rd-party batteries. If they don't work, you should be able to get a refund. While I agree that there's nothing much wrong with 3rd party batteries, I also think that I'm not going to buy more than one or possibly 2, so the difference may not matter so much.
My refurbished D80 came with a 3rd party battery, and I bought a second 3rd party battery for it. I sold it with one of the batteries, but the battery that I kept did not work in the D300s that I have now. It was a reputable make - Delkin Devices. I now have 2 Nikon batteries for the D300s.
Chris