Interesting. From the Oppo promo photoes it looks like the PM3 is single sided entry with a male trs plug wich only gives 3 wires, but I suppose that internally in the cup it might actually be a female trrs with 4 wires.
You certainly can get a balanced cable for the A1 but I'm not so sure with the PM3, looks like the balanced cables that Oppo sell are only for PM1 and PM2. For PM3 you might need to open them up and hardwire a new cable directly to the drivers like what I've had done with my Signature Pro
That depends on the cable that is fitted to phones from stock. If it runs 4 wires all the way to th plug you can just cut the existing trs plug of and fit a new trrs plug. Some phones however only run 4 wires to the Y join and then 3 wires from the Y join to the trs plug and others (typically those with one sided entry at the phone) only runs 3 wires all the way. In these cases you may need to open the phone and solder a completely new cable onto the drivers in order to go...
The headphone needs to be cabled and terminated for trrs which means 4 wires all the way from plug to drivers (2 wires for each driver). After that you can then have a pigtail thst takes it down to trs for normal single ended use.