rs485 between (3) slc5/02 with aic

I have a upcoming project that has a rs485 network between 2-slc500 5/2 via aic modules. What I would like to do is add a third with discrete output card in the rack that monitors points from the other two racks. What I planned on doing is writing in the N files to the new processor. Any suggestions would be appreciated.

Read up on Rslogix messaging. (MSG) in the SLC world.
This topic has been covered many times in the forum, just do a search.
Are you adding a 3rd processor only to grab data? If so, WHY?
Is it to control something and supply discrete outputs through your output card based on your other PLC's status?
There are other ways to do this........that may save you time and money....

It was going to monitor and give outputs at a remote location to be sent to the discrete inputs of a existing callout device.