How can I achive that I control, lets say, three arduinos with relay on the end. They will be communicating with nrf24l01, so there will be One TX and Three RX. I want to open/close relay on those three arduinos depending which one i choose. For exp. when I press button 1 of 3 it will ONLY open relay on first arduino.
How should I use pipes then?
You can achieve that assigning one specific channel to each slave Arduino. Then the master just have to select the right channel for the slave Arduino you want.
I don't know what library you use but if you look at RF24 you can use setChannel() API to achieve that.