Along about election time, the nation’s billboards blossom into a rogues’ gallery of aspiring politicians. This year, in California, at least three candidates succeeded in the spring primaries and headed for the fall election without resorting to billboard advertising. Whoever wins the congressional race between Paul McCloskey (Rep.) and Robert E. Gomperts (Dem.) in California’s nth District, and whether William T. Bagley (Rep.) goes ,to the State Assembly from the yth, the public is already ahead.