Shrug.  That doesn't really bother me.  x**y%z and pow(x, y, z) aren't 
going to match anyway, as soon as x**y has to be rounded.

What would bother me more is the idea of having, with precision 4:

pow(3, 22, 12347) -> nan
pow(3, 23, 12347) -> 7820
pow(3, 24, 12347) -> nan
pow(3, 25, 12347) -> 8645
pow(3, 26, 12347) -> 1241
etc.

(or a similar phenomenon with the 1st argument varying and 2nd and 3rd 
arguments fixed).  Or something that called pow(x, 3, 100003) with 
precision 5, and worked for all but 3 possible values of x.  That sort 
of thing is the cause of hidden bugs.

It's all academic really;  no-one's going to use Decimal's 3-argument 
pow.  I'm not really sure why it's supported at all.  Come to that, I 
don't really understand why Python's 3-argument pow belongs in the core 
interpreter (as opposed to somewhere in the standard library).  If it 
hadn't been there, and if __pow__ didn't accept a third argument, I 
suspect no-one would have tried to implement 3-argument pow for Decimal.