TMZ Composite

Abby and Brittany Hensel are a dynamic duo, and now they have a little buddy making 3 ... carrying around a baby in a small Minnesota town!

The reality stars and famed conjoined twins were in a parking lot Thursday in Arden Hills, MN -- just northeast of Minneapolis -- while they were putting a car-seated baby into a black Tesla. TMZ.com

The twins took their time here with the newborn ... securing the child in the back before closing the door and looking through the trunk. TMZ.com

We don't know much about Abby and Brittany's errand buddy -- whether it's theirs or someone else's. We've reached out to the pair and Abby's husband, Josh Bowling , but have not heard back from them.

As you know ... Abby and Brittany stoked conversation online last year when it

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