Israeli soldiers and Palestinian police exchanged gunfire at the main border crossing between their territories today. The incident followed another melee nearby on Monday night, when Israeli troops shot and killed three PLO officers. As many as four Palestinians -- including two police officers -- were wounded today after Israeli troops opened fire on a vehicle at the Erez crossing. Another PLO officer was wounded in a second clash nearby. In every incident, both sides insisted that the other had provoked the violence. The mounting violence threatens talks begun this week in Cairo over the question of extending Palestinian autonomy.