In an email obtained by 9to5Mac, it seems that Apple is informing its employees and education partners that the third gen Apple TV is being discontinued as it has reach the end of its life. They also mention that they will be fulfilling all shippable backlogs, meaning that once these units go out, there’s pretty much no more left.
“Effective immediately, Apple TV (3rd generation), MD/199LL/A, has reached end of life and is now discontinued. The Apple Supply team is going to fulfill all shippable backlog.” Now it is possible that you might still find some units available in Apple’s Retail Stores, depending on how many people are still buying it, so you can always try your luck there if you think that the third gen Apple TV is more than enough to suit your needs.