Note the order of maxsize and next, has to be specified in that order, a little annoyingly:
alter database datafile 'FILENAME' autoextend on next 1024m maxsize 3072m ; alter tablespace TSNAME add datafile 'FILENAME' size 1024m autoextend on next 1024m maxsize 3072m ;
From 10gR1 and above you can specify sizes in g. With 9i and below, you are stuck with sizes in m, like the above.