Hi Virgil, and a belated welcome to the forums - glad you could join us
Thanks for your interesting introduction - it's great to learn a little about new members!
Congratulations on your upgrade to the K-3. It's a lovely camera, and a perfect pairing with the 18-135 (I've never owned that particular lens, but it has many fans here). I'm a beginner when it comes to wildlife and BIF photography, so I won't offer any advice there, but there are plenty of members with lots of experience in these areas.
Regarding the D-AC50, you should be fine using that with the K-3. Many listings for third party equivalent products advertise them as compatible with the K-7, K-5, K-5II / s, K-3, K-3II etc.