Hi, Ken.
If I were to rate these 3 mikes, I'd put the Audix D6 first, followed by the AKG D112, then the Shure beta 52A.
The Audix D6 (for me) has a more honest sound and it's an easy mike to position and get a decent sound. In fact, it could easily be confused with the Sennheiser MD421, but the D6 would still win if I had to choose between both.
AKG's D112 isn't as good as their classic D12, but it's a popular mike for the kick drum. Personally I prefer it on some floor toms (but that's just me). What I don't like about this mike is you don't get as good a "snap" because of it's ice-pick high-end. Everything is far trickier position-wise and (of course) on the condition of the drum hardware - which makes me opt again for the Audix D6 or the Sennheiser MD421 because it's so easy to get a decent sound quickly irrespective of new or old bass drum hardware.
The Sure Beta 52A comes third place only because I haven't used it on a kick drum. Interestingly, it can be seen sometimes on TV -- used used on floor toms.... and... wait for it... under the snare drum. As I said, I've never used this mike myself so could only provide that obsevation. If John sings it's praises, that's good enough for me.
As Cary said, you'll be happy with any one of them.
Hope this helped a bit.
Rab
