Sprint has pretty much discontinued HTC EVO 3D on their network. It is no longer on sale from their website.
The remaining few units are available through the retail stores in the US market.
The company is replacing this model with the recently announced HTC EVO 4G LTE which is a pretty solid device in itself.
HTC EVO 3D was launched less than a year ago and this just goes on to prove that the devices have a very short life in the Android world.