The valve angle changes I'm referring to here are the angle of the valve's face in relation to the piston's flat surface. Way back when, in order to fit the wide 90-degree V-8 inside a stock engine compartment, OEM engineers angled the top of the valve stem in toward the intake manifold, compromising flow. Race engine builders then started angle-milling cylinder heads to move the valve stem back out toward the exhaust side, which puts the valve in a better relation to the piston. This produced a better-breathing cylinder head, and as such more power came along with it. But angle-milling the heads meant that you also had to move everything else to match.
Today, cylinder head companies are simply moving the valve angles within the heads, but the intake manifold and rocker arm companies have a tough time keeping up with the varying angles. And so far I've only talked about rolling the valves toward the motor's exhaust side.
Canted valve heads' intake and exhaust valves are at different angles, and splayed valve heads' valve centerlines point out from each other as viewed from the top of the heads. This allows large valves room to open in small cylinders. Actually, canted/splayed valve technology is nothing new. Big-block Chevys have had it since their birth, which might explain why a stock oval-port big-block head can flow just about as much as many race small-block heads. The most important thing to keep in mind here is that whenever you move the valve angles around, it's going to cost more money to build the engine.
This illustration highlights how complicated the end of a normal V-8 valve can become. Study it carefully. All these angles, widths, and dimensions can be modified for increased performance. It takes days, even years to develop the perfect setup, and only the guys with the biggest budgets have the resources to find minute differences in power here.
The three-angle valve job you've heard so much about refers to the angle of the seat cuts on the valve and in the head. Typically, the valve actually has only two of the angles (highlighted in the circles), and the head has the third, a transition cut commonly called a "throat cut," used to blend the valve seat into the cast area of the port's bowl. It does not have a corresponding cut on the valve face, and the valve will never touch this area.