A three-week truce in Bosnia neared the edge of collapse today, as fighting escalated in the northwest and both Bosnian Serbs and Muslims violated the terms of the agreement. Despite the Muslim government's claim that all its soldiers had been withdrawn form a demilitarized zone, U.N. inspectors found about 60 still hunkered down there. The Serbs refused to carry out their pledge to open a land route out of beseiged Sarajevo. Instead, they blocked movement of U.N. military convoys in much of the territory they control. Sunday, Serb shelling in Bihac, in northwest Bosnia, killed two teen-age girls and wounded 11 people.