Powering 3 DC motors (with limit switches) with AC-DC adapter
Connecting motors in series probably won't work anyway, because current draw is proportional to torque load so whichever one has the least load will hog the voltage. Also the same power is still required so it won't change the size of power supply required.

Next problem is that though the motors might draw 4-6A each while running, at startup they may try to draw up to 5 times that. Most large power supplies have fold-back current limiting to prevent damage if overloaded or shorted out. So you either need a power supply which can put out ~90A peak, or something else to handle the startup load.

Assuming the shelves will only be moved occasionally, my suggestion is to use a 12V SLA battery, with a 'standby' or 'float' charger to keep it topped up. A 7Ah battery should be large enough. Short circuit current could be well over 100A, so put a fuse in the battery circuit! Fusing each motor would also be a good idea.