George, thank you! Flipping it over and removing the bottom screws did the trick. It comes apart quite easily that way.
I found my leak, but there still seems to be something wrong with the pump for bath 3.
After I got the bottom separated from the top, I carefully flipped it back over and used some bricks and boards to raise the top black bath section up above the grey bottom shell a couple of inches. I filled all three baths. I used the priming funnels, as per the instructions to prevent air lock in the pump.
I saw my leak even before turning on the power. It's in the bath itself, not in one of the tubes. The step-down transformer in the lower right corner of your first and third picture has a white plastic part perpendicular to the windings. Each of the four corners of the plastic plate come to a curved point. The transformer twisted on its mount and one of those plastic points pressed into the end of the molded tank and put a hole through it. It was probably a casualty of shipping.
The number three pump comes on and circulates for anywhere from 2 to 20 seconds, and then it shuts off. There's a clicking sound, more like a clunk, when it shuts off. The agitation spray rod doesn't seem dirty or clogged. It sprays evenly and strongly across its entire width until the pump shuts off.
The pumps are magnetic drive pumps. What can go wrong with that kind of pump? Is there anything in blix or fixer (which is what would normally be in the 3rd bath for RA-4 or B&W processing) that could damage the impeller magnets?