Is this lens compatible with my camera? - 1

Is the Sigma 70-300mm f/4-5.6 DG nikon mount lens compatible with the nikon D3100?

Have you checked with Nikon or your owner's manual for the type of body mount you have?

Yes it is an "F" mount type lens.

It will fit, it will give autoexposure. You need to make sure that it is motorised if you want AF. Most DG lenses are motorised, but there are exceptions to the rule, so be careful.