CN-SMO 2008 Problem 1

Given a set $S=\{1,2,3,\ldots,3n\},(n\in N^*)$ , let $T$ be a subset of $S$ , such that for any $x, y, z\in T$ (not necessarily distinct) we have $x+y+z\not \in T$ . Find the maximum number of elements $T$ can have.