Thanks to all for your contributions to my success... This discussion led me to eliminate what was not
the problem with my Pro 3. Made me focus on another solution...
After trying to prime unsuccessfully and attempting several different possible solutions repeatedly for over 4 hours, I realized (at about 4:00 a.m. when I awoke thinking about this problem) that my problem was not air IN the filter. My problem was air entering INTO the canister.
What I saw at 4 a.m. in my horizontal but lucid state, was that air was entering the intake tube at a greater rate than the water was. I tried pouring water down the hose to fill it up, but that didn't work. It turns out that it was a simple fix--one that I had inadvertently caused--and it worked beautifully with the following first try. And, wouldn't you know, the answer is in the triangle "!" message after step D1 in the set-up instructions.
"When connecting [the] intake installation tube into the elbow connector, only push [the] intake receptacle into [the] tube half way."
What occurred in my installation is that when you push the tube too far in, it pushes onto the raised interior vertical strength protrusions, and the the O-ring no longer can seal out the air.
All you have to do is to pull the tube and elbow apart, check that the O-ring is smooth around (I replaced mine with the spare), and put it back together. After that, I poured water into both the in and out tubes, pumped the air out about 3 times, and VOILA! It now works super!!! --Rip