The Dell 3100 chromebooks are back in stock with Dell. CDW says they will have this coming week but get your orders in ASAP.
As for models of chromebooks, 3 in 20 is unfortunately not way off from the rates that I have seen in general. I have used chromebooks from Acer, ASUS, Dell, Samsung, HP, Google, Lenovo in decent numbers. All of them have had a noticeable fail rate with time and (ab)use. Plan for spares and/or a service contract. We have done will with about 10-15 spares and a warranty/accidental damage coverage with a large fleet of devices. Devices will need repair but having some spares leads to a seamless workflow. The cloud nature of Chromebooks also means the data loss and transfer during the process is pain free. This needs to be built into the cost of the devices.