The History Channel will air its show "Unarmed Combat" at least two more times, according to the cable network's web site.    The show, which explores many amateur sports including wrestling and boxing, looks into the rich history of ancient Greek and Roman sporting contests.    2002 U.S. World Team member and four-time NCAA Champion Cael Sanderson is also featured on the program.    The listed airdates are as follows. All times are Eastern Standard Time:    Monday, April 21, 8:30 p.m.-9:00 p.m.   Tuesday, April 22, 12:30 a.m.-1:00 a.m.    Check your local listings for specific channel information.