  We construct a string sigma-model which low energy limit describes the anti
de Sitter gravity and spin 3 massless fields in Vasiliev's frame-like
formalism. The model is based on vertex operators generating vielbein and
connection fields in the Mac Dowell - Mansoury - Stelle - West (MMSW)
formulation of gravity. The structure of the vertex operators is based on the
hidden symmetry generators in RNS superstring theory, realizing the isometry
group of the AdS space. The beta-function equations in the sigma-model lead to
equations of motion in the MMSW gravity with negative cosmological constant
with the AdS geometry being the vacuum solution. Generalizations for the higher
spin fields are analyzed and equations of motion for spin 3 fields in $d=3$ in
frame-like formalism are obtained in the low energy limit of string theory.
These equations correspond to those of $sl(3,R)$ truncated Chern-Simons action
based on higher spin algebra $hs(1,1)$.
