3AC Economy class coach contain 9 berths instead of the usual 8 in a bay and costs Rs.65 less than an ordinary 3AC coach. rest are the same. SMB is there in 3AC Eco coach. SMB is not there in 3AC coach of Duronto.
trust it clarifies your questions. by the way this 3AC Eco is likely to be introduced in some other trains at a later stage according to news reports.