before getting a general ticket,check the availability or charts of 3a...if availability exists,then get a first class ordinary ticket...which will be upgraded to that of 3a later...inform the onboard tte before boarding the train and only then you will be allowed to board...if tte does not permit you cannot...also,an unreserved express ticket may also be upgraded to that of 3a,but as per rules,first class ordinary ticket is to be bought to be upgraded to 3a