Thanks for the great response. One more question: I have an airport express but its an older one. I think it only goes up to 802.11(g). If the express is used to "extend" (wirelessly) one of the AE's, will that cause that AE to work at only G speeds or can the AE support the Express with G and still connect to other clients at N and AC speeds? Thanks again.
So, while Apple describes how to extend wirelessly, it should be avoided. When you extend wirelessly, the Express radio will split time between uplink and client connections. But, so too will the radio on the Extreme that it connects to. So it will cut speed and capacity by about 50% on both on the 2.4Ghz radio. You cannot specify which Extreme it will connect to, it will use whichever has a better signal at the time, which can change as you fire up the Microwave, wireless home phones, wireless keyboards\mice, or Bluetooth devices.
AP Extreme is a dual band access point. If the Express connects via 802.11g, it will do so on the 2.4GHz frequencies and not interfere with the 5GHz radios. The 2.4GHz radios will operate in g mode for all connections, not n (I believe). But, if you can add it as a roaming access point (via ethernet), the only 2.4Ghz signal operating in g mode will be the express, the other Extremes will run in n mode.
So, try relocating the Extremes first and avoid the 802.11g Express. The express that looks like a power adapter for a Mac is the first gen, it came in g, or Draft n models. The one that looks like a white Apple TV is dual band n.
If ethernet is not an option for the Express, consider a MOCA or Powerline adapter to run Ethernet over tour existing Coax (CATV Cable) or Power lines. The newer models can approach, or even exceed 1Gbps speeds, so don't go cheap. Also, if the Express is needed to fill the gaps, and it is in fact a g model, consider a refurb or clearance Express, I just did a Google search and found several for around $50.
My home is about the same size as yours, but on two levels. I have a Time Capsule (ac) in the middle of the ground floor, an Extreme (Dual band n model) in the middle of the upstairs, and an Express n model at the far end of the downstairs where I have a weak signal. My internet speeds everywhere in my home matches my ISP speed of about 70Mbps. I also have a 1st gen Express (dual band) that I use in client mode to enable Airplay to an aux connected speaker on the patio when I want to stream music out there. As a client, it doesn't really interfere with the rest of the access points, so that may be a way to repurpose that express.