Internet speed degrades after adding a third EX6100

fordem wrote:
I believe the EX6100 can be configured as an access point - wire it back to the main router and configure them on non overlapping channels - that should prevent them from interfering with one another

Wiring back to main router would be ideal because you don't have to share half the bandwidth repeating data to the main router over the wireless channel. But I suspect you went to a wireless extender because hardwiring is not possible.

Looking at the EX6100 manual, I didn't see a way to change channels on your EX6100 extender. (which I can do on my
R6250 router)
The internet address for the EX6100 manual:

http://www.manualslib.com/manual/703750/Netgear-Ex6100.html?page=25#manual

On page 25, the manual does tell how to change mode:

"Enable 20/40 MHz Coexistence (2.4 GHz). The extender 2.4 GHz WiFi network can run in either 40 MHz mode or 20 MHz mode. When this check box is selected, the extender uses 40 MHz mode unless a nearby WiFi network is using 40 MHz mode. If that happens, the extender uses 20 MHz mode to coexist with that network."

Try this on each extender to see if it helps.