how can I send command from the Master Controller (the one connected to the sensor) to switch on all the 3 relays on the other controllers via the network (when the sensor give +3.3v signal to GPIO)?

Thank you in advance.

(when the sensor give +3.3v signal to GPIO)

I think this means a digital High level.

master_controller.py

# importing packages
import sys, time, onionGpio
from socket import *

# using GPIO0 as input
gpioNum = 0
gpioObj = onionGpio.OnionGpio(gpioNum)
gpioObj.setInputDirection()

# sender
s = socket(AF_INET, SOCK_DGRAM)
s.bind(('', 0))
s.setsockopt(SOL_SOCKET, SO_BROADCAST, 1)

# reading the sensor once a second
while 1:
    state = gpioObj.getValue()
    if (int(state) == 1):
        for i in range(0,3):
            s.sendto(state, ('<broadcast>', 50000))
        sys.exit()
    time.sleep(1)

other_controllers.py

# importing packages
import sys
from OmegaExpansion import onionI2C
from OmegaExpansion import relayExp
from socket import *

# Relay Expansion device address is 0x20
addr = 0
channel = 0
relayExp.driverInit(addr)

# listener
s = socket(AF_INET, SOCK_DGRAM)
s.bind(('', 50000))

while 1:
    state, wherefrom = s.recvfrom(1500, 0)
    relayExp.setChannel(addr, channel, state)

Unfortunately I have no Relay expansion.
Please take a high resolution (macro) photo from its SMD area if you can and share with us.

You can use ssh to run command on another Omega.

Example:
sshpass -p "PASSWORD" root@REMOTE_IP COMMAND

PASSWORD - default is onioneer
REMOTE_IP - remote device address
COMMAND - app/script etc. to run on remote device (you can use parameters to enable/disable relay)