SATURDAY AS USUAL IS COMEDY night on cable. In Rodney Danger-field's The Really Big Show on HBO (Sat., Aug. 3, 11 P.M. ET), the host uses skits to introduce five lesser-known stand-ups (Sid Youngers, Hugh Fink, Bob Zany, Harry Basil and David Tyree). This quizzical quintet gives it their best shot, but none is as funny as Paul (My Two Dads) Reiser in Showtime's 3½ Blocks from Home (Sat., Aug. 3, 10 P.M. ET). The genial comic verbally bobsleds through a routine that takes in hot beverages, greeting cards, number overload and a variety pack of embarrassments.