I left rechargeable AA batteries in the charger for 3 times as much as it's recommended, that is for 36 hours. Can I still use them? Did it damage them?
It's very unlikely: all the modern chargers I've seen have an automatic cutout that prevents over-charging, by monitoring the batteries' voltage, and stopping the current when it reaches a certain level. As rechargeable batteries reach fully charged, the voltage rises relatively quickly, and the cutout gets activated.
If the batteries were exceptionally warm when you removed them from the charger, that could indicate a problem: but that problem would be shortened battery life, rather than anything more dramatic. Otherwise, you'll be fine. (as long as there's no visible signs of damage, such as a cracked casing or leakage of electrolyte)