Internet speed degrades after adding a third EX6100

Just my opinion, but, what you're attempting to do is perhaps the worst approach to expanding WiFi coverage - by using wireless repeaters, which, by design, must re-transmit on the same channel, you're creating an environment where there are multiple access points in close proximity, all on the same channel and all of the repeaters will attempt to re-transmit simultaneously - at some point they will start to interfere with one another. The normal recommendation is to have the devices on different channels, selecting from 1, 6 & 11, so that no two adjacent devices are on the same channel.