So many options! I think it could possibly be goutweed, although a lot of the pictures had a different leaf shape than this. The leaves do look like Japanese creeper, but they don't have any of that red tinge to them. If they're definitely not poison ivy, though, I'll probably let them be! They're growing in the very back of the yard which is heavily shaded and we struggle to get anything intentional to grow there, so we'd rather have regularly-trimmed weeds against the fence than bare dirt. But of course, I didn't want to hit these with the weed whacker if they were poison ivy!