Want to add a third RBS50 with wifi disabled

Thanks. Given that everything else is working very well I would rather spend the money in an additional satelite than mess around with the ORBI settings. My alternative here is to have an electrician drill under the floor to pass a ethernet cable directly from the device to the existing RBR50. This would have its own costs associated so adding a third RBS50 simply to connect these devices via ethernet cable would be a possibility, unless this can create additional issues.

My house is entirely made of brick with concrete floors (three floors). I have the RBR50 in the ground floor, a RBS50 on the first floor and another RBS50 on the second floor. The 30 feet rule does not seem to apply in my home given initially the RBR50 would only have a ambar connection to the first RBS50 satelite. In a straight line these were about 20 feet apart but the "open path through the stairs" (if this makes sense was some 45 feet. I had to move the RBR50 to a place where the signal "open path" is down to around 33 feet (and less than 20 in a straight line) to get a green conection. 

If I place the additional satelite where I am thinking, this would place it some 15 feet from the RBR50 and some 18 feet from the first RBS50.

Current Firmware is 2.5.1.16. 

I do not know the brand of the Network operator Router/Modem. The ISP brands the modem as their own and it does not have a "brand" anywhere visible. I would add a photo but for some reason I am on a Mac and when I upload a photo it returns an error.