This plant has come up in my garden and I'm not sure how it got there. It's perennial, fast-growing, with purplish stems and serrated leaves that come in threes. It seems to grow to about a foot and a half, forming a dense ground cover, When snapped, the stems have a pretty strong scent, similar to pine sap perhaps??

I like the plant, it seems impervious to deer and groundhogs, but does seem to be pretty aggressive....

and ideas? Thanks!