It comes down to bridging. I use discord and slack via IRC bridges. I actually use slack a lot (for work), but primarily through irslackd. I do not use slack for anything outside of work and would prefer to keep it that way.

For discord, I primarily use it through bitlbee-discord. With this bridge/gateway, I can actually chat on different servers at the same time, so I wouldn’t mind this for different communities if I had to.

Matrix is last because I don’t really have a good briding solution for it and it just seems clunkier than the other two for me.