A permutation group $G$ on $Ω$ is called a rank 3 group if it has precisely three orbits in its induced action on $Ω\times Ω$. The largest permutation group on $Ω$ having the same orbits as $G$ on $Ω\times Ω$ is called the 2-closure of $G$. A description of 2-closures of rank 3 groups is given. As a special case, it is proved that 2-closure of a primitive one-dimensional affine rank 3 permutation group of sufficiently large degree is also affine and one-dimensional.