Most of the time it takes to learn an organ trio will be spent working through the Seven-Step Method.
PRACTICE INDIVIDUAL PARTS
Begin by practicing the right-hand part of one section (such as section 1) on one manual. Focus on one item at a time, polishing each at a slow tempo:
Continue repeating the passage until it is easy to play it flawlessly 3-5 times in a row without much thinking. When you are ready, increase the tempo slightly (2-5 beats per minute) and repeat the process. Write down the tempo at which you decide to stop.
Repeat the process with the left hand on another manual, and then the pedal.