Is this lens good for my nikon D3100?

Http://www.amazon.com/Tamron-70-300mm-4-0-5-6-Macro-Digital/dp/B0012UUP02/
is it good? Is tamron better or sigma? Am i gonna be able to focus manually?

It will autofocus.

Whether or not it's better than the Sigma is open to debate - you probably wouldn't notice much difference whichever you used, but reading a couple of reviews won't hurt.

Yes it looks good to me. And it all comes down to price range just do as much research as you can before committing to buying a lens.

Yes, you can, and you will be able to focus manually. Across the board, I think the Tamron range of lenses is somewhat better than Sigma's, especially when it comes to the high-end models, but in this price bracket I wouldn't worry too much.
Given its modest price, don't expect miracles. Having said that, there will always be individuals who'll tell you that you must invest in $ 2, 000+ lenses to get something of a decent shot, which is absolute nonsense. Of course, more expensive lenses mean better image and build quality but unless you'll be viewing your pis with a magnifying glass, I'm sure the Tamron will do just fine. Happy snapping!