(Portland, OR)  --  The Portland Fire Bureau says arson has been determined to be the cause of three recent fires, but it's uncertain whether the cases are connected.  Two of those fires took place in Northeast Portland early yesterday morning, one of them at a home on Northeast Monroe Street that had been burned in August during a fire at an adjacent apartment complex that was under construction.  Also yesterday, a fire was reported at a vacant gas station on Northeast 7th Avenue.  A third arson happened at the corner of North Vancouver and Fargo on February 17th.