To clarify a bit, based on your original title: you cannot select the third row of a table, because sql tables are inherently unordered. What you can do (and is closer to what your query is asking) is to select the third row of a particular query, by using LIMIT as John said.
But even this has limits (sorry for the pun.) Let's say your table has 10 rows representing users, the first one created 1/2/2000, the next one 1/3/2000, followed by four users all created on 1/4/2000. There's no guarantee which of those four users will be row #3 of the results. In fact, the dbms would be perfectly within its right to return a different relative ordering of those 4 each time you repeated the query. Not likely, but not illegal either. Be careful what you ask for.
