Britain's Transport Department has issued a third warning to British airports and airlines to be on the lookout for terrorist bombs hidden in radio-cassette players like the one that blew up a Pan American World Airways jumbo jet over Scotland, killing 270 people. The telex Monday was issued as the U.S. and British governments were criticized for giving inadequate warnings of a possible terrorist attack before the Dec. 21 bombing of Pan Am Flight 103. The newest warning said the risk from booby-trapped electronic devices is "high and continuing."