Placing them in the attic area would provide sufficient coverage for the floor area directly underneath them, particularly if there's no insulation between the two. But Andy is correct that they will perform even better on the same plane (same floor). For your space, I wouldn't think that three would be necessary. My home is 3600 sq ft on three floors, and I have two routers set up on the top and main floor. the main floor serves the lower level quite well. Make sure they all support dual band, and .11AC if your Rokus are new enough to have that support.
One reminder: make sure the routers you buy have the ability to install as Access Points. You don't want them to act a firewalls between your network segments. That way they all get their IP addresses from the same DHCP server and are all able to see each other. If you install a router as configured out of the box, you could run into all sorts of network connection issues.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.