Realm Strategy approach

[Dan Rollo]

Hi,

I have a question about how to build a solution in shiro involving 3 
realms. Given realms A, B, and C, here are the rules.

A user must authenticate with either realm A or B.
A user must also authenticate with realm C.

Is their an existing strategy (or shiro.ini setup) that supports this 
sort of thing out of the box? Or should I create a custom strategy? 
Other approaches?

Thanks,
Dan