Mayor Angel Taveras said he is raising money for a nonprofit to hire additional street workers as the number of homicides in the city is triple what it was at this time last year. The mayor said Thursday that he will lead a fund-raising effort so the Institute for the Study & Practice of Nonviolence can increase the number of its street workers from four to 10. The number of homicides in the city now stands at 12 this year, up from four at the same time last year. There were 12 homicides in the city during all of 2011. Three people were shot to death in a city apartment this week, and three young children were in the apartment at the time of the killing.