Possible Duplicate:
Is there a free subgroup of rank 3 in $SO_3$?
How is this possible?
Here are some facts:
$F_2 \subset F_3$
$F_2 \not\cong F_3$
Fact 2 implies that $F_3$ is isomorphic to a proper subgroup of $F_2$, i.e. one in which there are exist some relations between the generators of $F_2$.
But I'm really struggling to come up with anything else.