The propability of you having 3 speedo's in row not working coming right from Novak is highly unlikely... Keep in mind that they are all tested before they are sent out...
You must be doing something wrong. Make sure that the black wire is facing the oustside of the receiver when you plug it in. It will plug in backward perfectly fine but then you will have no power.
I would call Novak and talk to a tech or utalize the Steve Wiess thread with RCtech.