  The three 3-brane system with both positive or negative tension is studied in
a low energy regime by using gradient expansion method. The effective equations
of motion on the brane is derived and in particular we examine, in the first
order, the radion effective lagrangian for this system. In this case, we show
the solution of the modified Friedmann equation with dark radiation on the
middle brane and the other 3-branes by direct elimination of the radion fields
and Weyl scaling of the metric on the branes. We also derived the scalar-tensor
gravity on the branes.
