I'm building blast gates.

I need to cut some clean 6-1/4" (or thereabouts) holes. Ideally, I'd
cut through the 3/4" + 1/4" + 3/4" at the same time to get a perfect
fit.

I'm guessing the router is the way to go, but can you lead me to some
hints and tips, pictures, or other suggestions?

Thanks,