We first present an equivalent definition of relative generalized Hamming weights of a linear code and its subcodes, and we develop a method using finite projective geometry. Making use of the equivalent definition and the projective-geometry method, all of the relative generalized Hamming weights of a 3-dimensional q-ary linear code and its subcodes will be determined. Key words. generalized Hamming weights, relative generalized Hamming weights, support weight, relative difference sequence AMS subject classification. 94B05 DOI. 10.1137/090770254