2 ac side lower berths are slightly broader than 3 ac seats. thats all. 2ac side berths have curtains while 3ac does not have curtains. except the breadth, not much difference.
if you look at the total bay, then there are 6 seats in 2ac i.e. 4 + 2(side)
while it is 8 (6 +2side) or 9 incl. sMB in some coaches of 3ac. trust this clarifies.