The Alpha 12 could certainly be used in a three-way configuration. It works quite nicely in a π cornerhorn, so you could surely put a midhorn and compression tweeter on toip of that. Adrian's Alpha 6 midhorn would work well, as would a larger midhorn using an 8" or 10" driver.

The thing is that the Alpha 12 is really designed to be a full range speaker. It's best applications, in my opinion, are on infinite baffles or very large boxes. Its best feature is that its impedance curve is benign so there isn't much interaction with the output circuit of a low power SET with no feedback. Those are sensitive circuits, but the Alpha's don't cause problems for them. That, plus their low cost, make them ideally suited for two-way speakers used on SET amps. But the three-way idea you're looking at would be good too.

Wayne