It dependes on the stacks, your energy system and your belief system.
Can be different topics, can be same topic, can be complimentary topics.
Just try it out and see how it works for you.
If in the evening you already get tired after listening to one stack for the whole day then I would not add another one.
Use the Chakra audios and Soul Restoration series, as well as Mana Circuits and Auric Repair to extend your energy system over time. Also “Raise Your Vibrational State”.
Yes, if you can play them from 3 different devices.
Never play two fields at the same time from the same device.
2 stacks at the same time = 2 devices
3 stacks at the same time = 3 devices
etc.