Communication between three arduino megas

I hope I have the right forum section for this question.
I have an arduino mega (master) monitoring 18 pushbuttons on a model train layout. I want to then send to one of two other arduino megas the number of the pushbutton that was pressed, have that arduino then operate an SG90 servo to change the point (turnout), read the microswitches and send a message back to arduino master that the point was successfully changed. Arduino master then changes the LED for that point on the display panel.
I would like to use HCPCA9685 modules to operate the servos (one 9685 attached to each secondary arduino mega}.
What can I use to communicate between the master mega and the two secondary megas and still leave the i2c free to operate the PCA9685's?
Distance between master mega and other megas is about maximum 6 metres.
I have considered the nRF24L01 units and the HC-12 units but I am not sure if the distance is too large or too small.
Hope my description is not confusing and I can tap into the experiences of members of this forum.

For maximum 6 meters distance, I think RS-232 is a good choice. You can use Modbus RTU over RS-232
If you want to use wireless, you can use Modbus TCP.
The Modbus protocol supports multiple masters - slave, which suitable for three Arduino Mega.
You can learn more about Arduino - Modbus

The Mega has four hardware serial ports. I don't understand why you would not consider using them - with plain-vanilla serial. The NRF24 is good for about 10m, dirt cheap, made for this sort of thing, and saves on cabling. I understand I2C is only for very short distances, and not worth considering for this purpose, but it is a bus and should not pose the problem you are implying.

search for RS485, use RS485 modules, introduce a BUS.

The nRF24L01+ modules would work well for a range of 6 metres and avoiding wires has a lot of attractions. I use an nRF24 on a Mega to control radio-controlled 009-sized model trains.

Have a look at this Simple nRF24L01+ Tutorial.

Note that the SPI pins on a Mega are in a different place from an Uno - see the SPI documentation

Wireless problems can be very difficult to debug so get the wireless part working on its own before you start adding any other features.

The examples are as simple as I could make them and they have worked for other Forum members. If you get stuck it will be easier to help with code that I am familiar with. Start by getting the first example to work

There is also a connection test program to check that the Arduino can talk to the nRF24 it is connected to. If the first example does not work be sure to try the connection test for both of your Arduinos. Nothing will work if the connection test fails.

A common problem with nRF24 modules is insufficient 3.3v current from the Arduino 3.3v pin. This seems to be a particular problem with the nano. The high-power nRF24s (with the external antenna) will definitely need an external power supply. At least for testing try powering the nRF24 with a pair of AA alkaline cells (3v) with the battery GND connected to the Arduino GND.

If you want two-way communication the second example (using the AckPayload feature) is probably the simplest. It's what I use with my model trains.