Kevint,
Thanks for the additional info - you already have several people crunching numbers for possible projects!
The 3' diameter pipe may be extreme for "normal" flow, but that pipe may also serve the function of your emergency spillway.
Does that 3' drain pipe run full bore for a little while after you get a "frog strangler" rain? If so, then you need to plan for your mechanical devices to be protected when your pond goes into flood stage.
You could install a small siphon pipe with an automatic shut off when the water level gets too low. That might allow you to "right size" your pipe flows to exactly match the mechanical requirements of any device that you install.
