Traditionally, the term "three angle valve job" refers to three separate angles on the seat. Most already know this, but I'm going through the "exercise" for those that may not be clear. The "top" angle is 30 deg. The "seat" is 45 and the "throat", 60. As a youngster in machine shops, that's the way we were taught. The primary purpose of the distinct angles is to insure the seat angle is concentric and of equal width all the way "around". The intake seat isn't as crucial regarding the width of the seat for longevity. The exhaust is very important, as that's how the heat dissapates from the valve. Heat is like electricity. It follows the path of least resistance. A "thnner" portion will transfer heat in a more concentrated manner, "burning" either the seat or the valve.
Today, we use a little different rationale on the angles. The seat angle of a typical small block will always be 45 deg. But by changing the top and throat, we can improve flow. A 15-17 deg. top and 70 deg throat will significantly increase flow "across the lift range". To the tune of 5-7 CFM. The intake is more "reactive". A top of 10, blended to a 30, a seat of 45, .060" wide and a 70 deg throat will improve mid- and high-lift flow. A wider seat and "more" 30 will improve low-lift flow. For a lower revving street engine, low-lift flow is important for torque production.
All that said, the "provided" valve seats will seal the cylinders and allow the engine to run properly. They are, by no means, "optimum". Some of the Dart and Brodix stuff I've seen delivered "ready to run" had really nice radius seats.
That brings us to modern "high performance" or "competition" valve grinds. Most shops today use stones only for "stock" valve jobs or "truing" the seat angles after the cutters do their "stuff". We have carbide cutters in various "profiles" for different applications, operating in a heavy machine. Very accurate. A true radius (exhaust) will obviously flow much better than anything with abrupt changes. Many builders use a full radius cutter on small blocks for drags. We use one that has a 15 degree "top" angle included in the profile for accurately limiting the OD. We also have a method of creating a "venturi" seat. Pontiacs REALLY "like" that one...
One last thing. Most engine families have cylinder heads based on different principles and applications. No two respond the same to the same seat profiles. Even some within the same family will respond differently to similar modifications. There are NO "absolutes".
This is strictly about the seats. There are some things that can be done with the valves, as well. That's a "story" for another day.