We've had our Big Agnes Fly Creek 3 out in about 6 rainstorms of 2 to 4 hours each, with no problem. But on our last trip we had quite a drencher and ended up with water in the tent. It appeared to be leaking along the seam at the top of the fly.
I checked the seam tape and it looked good. I picked at it a bit and it felt good. Then I pushed the seam into a 20 ounce plastic cup, filled it with water and lifted it out.
If you have a FC3, I suggest you do the same.