Ah, I see, I read it as draining in between. Still, if you are not stirring to homogenize the mash after you add the last third, I think it is close enough to sparging.
Like I say above, if you are just topping it up and running it out the bottom without stirring then I think you’re sparging. If you stir it in then I’d agree, it is a step infusion.
If you are trying to increase your efficiency, take a look at your crush; especially if you batch sparge. I batch sparge and hit almost exactly 75% efficiency every time. I don’t brew much above 1.075 gravity though or I suspect it might drop.