BEIRUT - Lebanon's army on Saturday fired at a vehicle carrying Islamist militants, killing three of them and wounding two others in the north of the country near the Syrian border, a security source said.The source said it was unclear which group the militants belonged to. They were driving near the outskirts of the town of Arsal, the source said, without providing further details.Islamist groups including al-Qaida's Nusra Front have a presence in western Syria, including near the border with Lebanon, and there are have been numerous incursions by Islamists reported in the porous border region.