Don't know if you still need help since this question is 5 months old, but- Is it three triangles upside down? One on top, two on the bottom? Possibly with three circles, too?
If I found this in my yard I would think it was done by people trying to be "Satanic cult"-like since people are always associating inverted stuff with Satanism and then scaring people with it. Not saying these people aren't possibly dangerous, I'm more implying that they're probably not dangerous in the magic sense, so I'd be more concerned with guarding your house physically.
Hopefully you haven't been having weird and menacing things happening in your house. If you have though, I'd consult with a religious leader of your religious or spiritual path, if you have one, or simply finding one you trust and getting your house blessed. IMO, symbols are pretty much symbols unless used with intent, so if this one is causing bad stuff to happen and you use another symbol to counteract it, it probably won't do anything unless you believe in it, so doing/using something that you believe in is key.