As Memorial Day weekend wore on, by Sunday morning Chicago police had responded to the shootings of 21 people, three of whom died from their injuries, officials said.The grim tally was only expected to increase as a shooting Sunday about 6 a.m. in the 1300 block of South Throop Street in Englewood is said to have left two dead and two injured. The shooting was possibly in retaliation for an earlier one in the same neighborhood on West Hastings Street, which also left a man dead, investigators said.