Unless you have an older 90mm I'm unfamiliar with, the Tamron 90mm will work with his D3100 per it has a focus motor. Tried it out on my D5100 after playing with it on my D7100.
This lens: Tamron SP 90mm f/2.8 Di 1:1AF Macro Lens for Nikon AF AF272NII700
Doing a littler further digging - it looks like at one time it did not have the focusing motor. It does now though.
I do have the older version... I stand corrected then. Didn't know the newer model had a focus motor. Although I use it on my d600 so that doesn't matter. I do however manual focus on macros. More accurate
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