ORBI RBK50 placement in a large multi story house

Unless you got a "great deal" buying three RBK50's was not the optimal move.  Yes, an RBR50 can be attached to an Orbi system as an Access Point, but it is a separate WiFi system, not integrated into the primary Orbi.  WiFi devices which never move, of course, will not care.  Once attached, they will stay attached.  Both of the extra RBR50's can be made into stand-alone access points.  It would have been more efficient to get the RBK53, which has one RBR50 and two RBS50 satellites.  The optimal solution was to get ONLY more satellites.  Alas, Netgear's distrubution channels make this often frustrating and users do as you did and buy what is available.

Where to put the units depends on what kind of coverage results based on the building construction.  Start with the router unit where the ISP modem is.  If the entire ground floor gets good signal, move to the 1st floor.  If not, then deploy one or more satellites to cover the ground floor before moving up.  Repeat on the 1st floor, and then the penthouse.  Those exterior security cameras being "fixed" are ideal targers for the surplus routers.

p.s. If the ISP modem is actually a "modem/router" then you have a "double NAT", which is NOT GOOD.  Either the ISP unit or the Orbi router should be changed.  We die-hard Orbi owners prefer to put the ISP unit into "passthroug" or "bridge" mode and keep the Orbi as the only router.