Jean Shepard, fishing tor folksy American humor in a boat in the middle of the Okefenokee swamp, catches the essence or his new public television series in one quick moment. "Well friends, this ain't 'Three's Company,"' he says dryly. No, it certainly ain't Not all television viewers will be hooked by "Jean Shepard's America." but the half-hour episodes of Shepard's musings about things universal are undoubtedly unlike anything available on the small screen these days. . .