Tamron 90mm Macro would pair well with your 3100. If your budget allows, the Nikon 105 VR Macro is the gold standard.
The Tamron will perform maybe 85% as well for 40% the money. I'm no pro, those numbers are a bit of a guess but I bet not too far off.
All that being said, I played with them both on a D7100 like mine at the store the other day, and there's no denying that the Nikon looks and feels like a MUCH higher quality piece of kit. Maybe overkill for a novice like me though, or for the D3100 body.
If I were smart, I'd buy the Tamron.......