what is the difference between 1st ac 2nd ac and 3rd ac in train?
The numbers 2 or 3 spell out the difference: there are 3 berths on each side (but, mercifully, no longer on side berths) whereas 2 for 2AC. You also get marginally better trim and sometimes a bit better food in 2AC versus lower-cost 3AC. Fare of 2AC is typically 140% of 3AC fare.