On PMX-A series, the external signal common (ACOM) for the output voltage monitor is assigned to the pin No. 3, 5 and 15 on the J1 connector. Which one should I use?

Since ACOM is the common and all ACOM (pin No. 3, 5 and 15) are tied together, you can choose any one of them.
For the output voltage monitor, the pin No. 3 would be suitable because it is closest to the VMON terminal (pin No. 1).