Hello bkelly0123. Welcome to the Apple Discussions!
1. Am I losing bandwidth by extending the wireless network, or does that issue only apply when connecting only wirelessly (NOT using an Ethernet connection between the two AEBS units)?
Yes. Bandwidth loss is common when extending the AirPort with another by wireless. Since you already have them connected to an Ethernet backbone, you want to configure them for a "roaming" network and not for a Wireless Distribution System (WDS).
2. Can I "extend a wireless network" to a third AEBS unit in another part of the house (near the patio for better wifi outside)?
Yes, but not with the newer dynamic WDS. Instead, you would need to configure the three base stations into the older static WDS configuration. This type of WDS supported three types of base station configurations: main, relay, & remote. It also only operates in the 802.11g mode with significant bandwidth penalties.
What is the limit on the number of wireless devices I can use to extend the wireless network -- assuming all are networked via Ethernet?
Basically, an unlimited amount. However, is relative tight quarters you would need to take into consideration of the radio channel that each base station would operate on as to not conflict with the next one closest to it.
3. Am I extending both the B/G and N bands (since I am using two AEBS simultaneous dual-band units)?
You would be extending both the 2.4 GHz & 5 GHz radios. Note: 802.11n can operate on either radio; while 802.11b/g can only operate on the 2.4 GHz band.
Again, since you already have an Ethernet backbone, you will NOT want to use a WDS. Instead, you will want to configure your AirPorts for a "roaming" network. In this type of network, you can literally "roam" with a laptop from room to room and still be on the "same" wireless network.
The following is the basic steps to configure a "roaming" network:
To setup an 802.11n AirPort Extreme Base Station (AEBSn) as a roaming network:
Network configuration: DSL/Cable Modem or Internet Router > (Ethernet cable) > [WAN] AEBSn#1 [LAN] > (Ethernet cable) > [LAN] AEBSn#2 [LAN] > (Ethernet cable) > [LAN] AEBSn#N
Setup the AEBSn connected to the Internet to "Share a public IP address."
Internet > Internet Connection > Connection Sharing: Share a public IP address
Setup the remaining AEBSn(s), as a bridge.
Internet > Internet Connection > Connection Sharing: Off (Bridge Mode)
For each AEBSn in the roaming network:
o Connect to the same subnet of the Ethernet network
o Provide a unique Base Station Name
o The Network Name should be identical
o If using security, use the same encryption type (WEP, WPA, etc.) and password.
o Make sure that the channel is set at least three channels apart from the next AEBSn.