We study internal selective reflection from a dielectric-vapor interfa
ce, where three levels of the vapor atoms in a LAMBDA configuration ar
e connected by two light frequencies. The reflectivity contains then b
oth a sub-Doppler structure due to spatial dispersion in the boundary
layer and the dark resonance arising from coherent population trapping
. We derive analytical expressions for the reflectivity of either one
of the two light beams. The contributions from atoms approaching or le
aving the interface are strongly different, whereas they are identical
for the case of two-state atoms.