I wouldn't be so quick to blame the router. I had a friend give me an ATV3 that "suddenly" stopped connecting to the wifi. The problem is exactly as described by the OP. I downgraded to two previous firmwares and same problem. My ATV2's work no problem it is just the ATV3 that is acting up. This thread describes the problem and the Multiple people experiencing the same issue.
It seems to me to be a problem directly associated with the ATV 3.