Obviously, Tchaikovsky, Chopin and Liszt were direct influences on Rachmaninoff.
But while Tchaikovsky's influence was on the overall style of Rachmaninoff, I think Rachmaninoff's second sonata is a direct descendant of Chopin's second sonata. It's just that I see no such connection between Rachmaninoff concerti -especially 3rd- and any other piece of music. Maybe the intermezzo of Rach 3 and the slow movement of Tchaikovsky 1, but I think that's done unconsciously, like the theme of the second movement of Rach 4 and Schumann PC. Though pianistically, I don't find Tchaikovsky interesting, neither do I think Rachmaninoff himself did. There is always an influence of Chopin and Liszt, which is inevitable, within a unique Rachmaninoffesque texture.