SAO PAULO — Brazilian police used tear gas, rubber bullets and stun grenades to disperse striking workers inside a subway station on Friday, adding to fears that labor unrest could disrupt the World Cup that opens here in six days. At least three union members were injured in the Sao Paulo clash, said Paulo Iannone, a union spokesman. Operators of the subway and overland trains were in the second day of a strike for higher wages, with no indication that it would end soon.