how to know the channel i receive information from when i use nRF24L01 to connect between 3 of them
The nRF24 is highly versatile and can be programmed for a number of different network topologies.  It appears you want either


One-to-many network - where the master node sends data to the slave nodes
Many-to-one network - where the master receives data from many slaves.


Both networks are achieved with the same radio channel (RF_CH) on all nodes. 

For a one-to-many network you need to have a different RX address for each slave node.  Set the master TX address to the RX address of the corresponding slave  that you want to receive the data.  Done.

For a many-to-one network you are limited to 6 slave nodes.  On the master radio you will need to set the addresses of data pipes 0-5 to the slave TX addresses.  The master will be listening for data on all pipes and signal an interrupt whenever data is received on any of them.

To achieve bi-directional communication with a (practically) unlimited number of slave nodes, your best bet is to use the in-built ACK PAYLOAD feature of nRF24.  In addition to configuring a one-to-many network, you need to enable ACK payload on each radio node.  The procedure to follow is given in the nRF24L01 data sheet.  I can't recommend strongly enough how important that is to read and learn from.  You will be able to send a message from the master to one slave, and get data back from that node in the same radio transmission, auto-retransmit etc included in the bargain.  This way you also know which node the data are coming from - without needed to encode the identity of the slave node in the message.  It's incredible what these little radios will let you achieve!