You can fill through whatever hole you want, but it doesn't show you when you're full like the actual fill hole. When you drain the trans in the car, I'd wager there's still a half quart in the trans after. When you open one up on the bench that's been previously drained, that's about how much comes out in my experience. That being said, 3 full quarts is probably going to be well overfilled, which tends to lead it blowing out the breather and making a mess, so no, no benefits.

I use a hand pump though the fill hole and just pump until it starts running out.