LONDON—Police and politicians in Northern Ireland appealed for calm on Monday after a third night of violence that saw Protestant youths start fires and pelt officers with bricks and gasoline bombs. The flareups come amid rising tensions over post-Brexit trade rules for Northern Ireland and worsening relations between the parties in the Protestant-Catholic power-sharing Belfast government. The Police Service of Northern Ireland said officers were attacked in Londonderry on Sunday night, and there was also unrest in two pro-British unionist areas near Belfast. Police said most of those involved were teenagers. Chief Superintendent Darrin Jones condemned the “senseless and reckless criminal behavior that [does] nothing but cause damage to the community.” The disturbances followed unrest Friday and Saturday in unionist areas in and around Belfast and Londonderry, also known as Derry, that saw cars set on fire and projectiles and gasoline bombs hurled at police officers. Police said 27 officers …